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Direct Reads and Smart Scan with Exadata

Posted by Jim Brull on Wednesday, June 13th, 2018 in Oracle Cloud

Direct Reads and Smart Scan

The most significant performance feature of Exadata is generally agreed to be smart scan. There are several prerequisites for smart scan – most notably, the query must perform direct reads.

The widely accepted (and published) stance on direct reads is that:

– They take place for parallel operations (parallel direct reads) and some full-scan operations (serial direct reads)

– “Full scans” includes full table scans and full index scans

– Full scans qualify for direct reads if the number of blocks being scanned exceeds a threshold

– This threshold is defined by “_small_table_threshold”, a dynamically adjusted initialization parameter that computes to 2% the size of Oracle’s buffer cache

– It’s published in many sources that direct reads happen when the number of blocks for an object exceeds (“_small_table_threshold”) * (5)

There are number of factors that can add variability to this formula – in addition to the size of the segment being scanned, the percentage of segment’s blocks in the buffer cache also plays a role, as does the number of dirty blocks in the cache. Oracle makes a decision at run-time to do direct reads or buffered reads and calls this “adaptive direct reads”.

Since direct reads enable smart scans on Exadata, it’s important to understand what makes direct reads “successful” or unsuccessful.

In this blog I’ll show a bunch of direct read and smart scan test cases. I’m doing this mostly because of this – the “_small_table_threshold” * 5 formula actually hasn’t quite worked out as expected based on what all the blogs, books, and documentation say. Hopefully these cases can stand on their own, but if anyone finds anything “off” with them let me know …

And before getting started, a couple of great references:

http://kerryosborne.oracle-guy.com/2010/06/exadata-offload-the-secret-sa…

http://oraganism.wordpress.com/2011/06/27/fun-with-_serial_direct_read/

http://afatkulin.blogspot.com/2009/01/11g-adaptive-direct-path-reads-wha…

http://dioncho.wordpress.com/2009/07/21/disabling-direct-path-read-for-t…

http://oraclue.com/2009/07/17/direct-path-reads-and-serial-table-scans-i…

http://oracledoug.com/serendipity/index.php?/archives/1321-11g-and-direc…

(All test cases are done on either Oracle 11.2.0.3/Exadata or Oracle 11.2.0.2 on non-Exadata OEL)

Smart Scan Test Case #1: Full-scan XLA_AE_LINES

XLA_AE_LINES is an Oracle EBS R12 table that’s typically large in almost every R12 environment of any respectable size. My test environment is not large at all, but I wanted to show a few smart scan test cases against it:

#######################

Test Query

#######################

APPS @ visx1> @exq.sql

APPS @ visx1> select count(*) from

2 (select code_combination_id,

3 sum(entered_dr), sum(entered_cr)

4 from xla_ae_lines

5 group by code_combination_id);

COUNT(*)

———-

5573

1 row selected.

APPS @ visx1> set echo off

PLAN_TABLE_OUTPUT

————————————————————————————–

SQL_ID ct0t92t7f0a99, child number 0

————————————-

select count(*) from (select code_combination_id, sum(entered_dr),

sum(entered_cr) from xla_ae_lines group by code_combination_id)

Plan hash value: 3427813926

—————————————————————————————————————

—————————————————————————————————————

| 0 | SELECT STATEMENT | | | | 4245 (100)| | | |

| 1 | SORT AGGREGATE | | 1 | | | | | |

| 2 | VIEW | | 4542 | | 4245 (29)| 00:00:01 | | |

| 3 | HASH GROUP BY | | 4542 | 49962 | 4245 (29)| 00:00:01 | | |

| 4 | PARTITION LIST ALL | | 6645K| 69M| 3369 (10)| 00:00:01 | 1 | 22 |

| 5 | PARTITION RANGE ALL | | 6645K| 69M| 3369 (10)| 00:00:01 | 1 | LAST |

| 6 | TABLE ACCESS STORAGE FULL| XLA_AE_LINES | 6645K| 69M| 3369 (10)| 00:00:01 | 1 | 610 |

—————————————————————————————————————

#######################

Smart Scan Stats

#######################

NAME VALUE

——————————————————-

cell physical IO interconnect bytes 111,206,400

cell flash cache read hits 1,312

CPU used by this session 237

#######################

Smart Scan Efficiency *

#######################

no rows selected

#######################

Waits

#######################

STAT TIME_WAITED TOTAL_WAITS

————————– ———– ————

events in waitclass Other 200 5

cell multiblock physical read 97 751

cell single block physical read 44 904

SQL*Net message from client 6 26

gc cr multi block request 6 238

gc cr grant 2-way 3 394

row cache lock 3 893

Summary from above:

– The query did a full-scan on XLA_AE_LINES with a PARTITION RANGE ALL operation, since we selected data from all partitions

– We used 237 CPU units

– There was no evidence of smart scan, based on the fact that there were no bytes eligible for predicate offload and because our IO-related waits were “cell multiblock physical read” and “cell single block physical read”

Smart Scan Test Case #2: Full-scan XLA_AE_LINES, Force Serial Direct Reads

For this test, I’m going to set “_serial_direct_read”=TRUE prior to running the same query and attempt to get a smart scan:

#######################

Test Query

#######################

APPS @ visx1> alter session set “_serial_direct_read”=true;

Session altered.

APPS @ visx1> @exq.sql

APPS @ visx1> select count(*) from

2 (select code_combination_id,

3 sum(entered_dr), sum(entered_cr)

4 from xla_ae_lines

5 group by code_combination_id);

COUNT(*)

———-

5573

1 row selected.

APPS @ visx1> set echo off

PLAN_TABLE_OUTPUT

————————————————————————————–

SQL_ID ct0t92t7f0a99, child number 0

————————————-

select count(*) from (select code_combination_id, sum(entered_dr),

sum(entered_cr) from xla_ae_lines group by code_combination_id)

Plan hash value: 3427813926

—————————————————————————————————————

—————————————————————————————————————

| 0 | SELECT STATEMENT | | | | 4245 (100)| | | |

| 1 | SORT AGGREGATE | | 1 | | | | | |

| 2 | VIEW | | 4542 | | 4245 (29)| 00:00:01 | | |

| 3 | HASH GROUP BY | | 4542 | 49962 | 4245 (29)| 00:00:01 | | |

| 4 | PARTITION LIST ALL | | 6645K| 69M| 3369 (10)| 00:00:01 | 1 | 22 |

| 5 | PARTITION RANGE ALL | | 6645K| 69M| 3369 (10)| 00:00:01 | 1 | LAST |

| 6 | TABLE ACCESS STORAGE FULL | XLA_AE_LINES | 6645K| 69M| 3369 (10)| 00:00:01 | 1 | 610 |

—————————————————————————————————————

#######################

Smart Scan Stats

#######################

NAME VALUE

———————————————————————- ——————–

cell physical IO bytes eligible for predicate offload 87,965,696

cell physical IO interconnect bytes 74,826,560

cell physical IO interconnect bytes returned by smart scan 67,756,864

cell flash cache read hits 832

CPU used by this session 196

#######################

Smart Scan Efficiency *

#######################

Eligible MB SS Returned MB Pct Saved

——————– ——————– ———

84 71 14.93

#######################

Waits

#######################

STAT TIME_WAITED TOTAL_WAITS

———————————– —————- —————-

SQL*Net message from client 49 28

cell single block physical read 42 835

cell smart table scan 27 445

events in waitclass Other 24 72

row cache lock 5 873

gc cr grant 2-way 3 357

enq: KO – fast object checkpoint 1 134

Summary from above:

– The query again did a full-scan on XLA_AE_LINES with a PARTITION RANGE ALL operation, since we selected data from all partitions

– The query was offloaded, as evident by “cell physical IO bytes eligible for predicate offload” statistics and a smart scan efficiency, which is calculated from

V$SQL.IO_BYTES_ELIGIBLE_FOR_PREDICATE_OFFLOAD > 0

– Further, we’ve got waits for “cell smart table scan”, which is one of the things we’d see for smart scans

– We also have a wait for “enq: KO – fast object checkpoint” – this happens when blocks exist in the buffer cache that need to be flushed for the data being read into the PGA to be consistent

Smart Scan Test Case #3: Full-scan XLA_AE_LINES, Parallel Query

Below, I’m going to parallelize the same query with a degree of parallelism = 8, without forcing serial direct reads as in the previous case:

#######################

Test Query

#######################

APPS @ visx1> @exq.sql

APPS @ visx1> select count(*) from

2 (select /*+ parallel (8) */ code_combination_id,

3 sum(entered_dr), sum(entered_cr)

4 from xla_ae_lines

5 group by code_combination_id);

COUNT(*)

———-

5573

APPS @ visx1> set echo off

PLAN_TABLE_OUTPUT

————————————————————————————–

SQL_ID cmpbrw3vw826m, child number 0

————————————-

select count(*) from (select /*+ parallel (8) */ code_combination_id,

sum(entered_dr), sum(entered_cr) from xla_ae_lines group by

code_combination_id)

Plan hash value: 4167058233

————————————————————————————————————————————————-

————————————————————————————————————————————————-

| 0 | SELECT STATEMENT | | | | 573 (100)| | | | | | |

| 1 | SORT AGGREGATE | | 1 | | | | | | | | |

| 2 | PX COORDINATOR | | | | | | | | | | |

| 3 | PX SEND QC (RANDOM) | :TQ10001 | 1 | | | | | | Q1,01 | P->S | QC (RAND) |

| 4 | SORT AGGREGATE | | 1 | | | | | | Q1,01 | PCWP | |

| 5 | VIEW | | 4542 | | 573 (27)| 00:00:01 | | | Q1,01 | PCWP | |

| 6 | HASH GROUP BY | | 4542 | 49962 | 573 (27)| 00:00:01 | | | Q1,01 | PCWP | |

| 7 | PX RECEIVE | | 4542 | 49962 | 573 (27)| 00:00:01 | | | Q1,01 | PCWP | |

| 8 | PX SEND HASH | :TQ10000 | 4542 | 49962 | 573 (27)| 00:00:01 | | | Q1,00 | P->P | HASH |

| 9 | HASH GROUP BY | | 4542 | 49962 | 573 (27)| 00:00:01 | | | Q1,00 | PCWP | |

| 10 | PX PARTITION RANGE ALL | | 6645K| 69M | 467 (10)| 00:00:01 | 1 | LAST | Q1,00 | PCWC | |

| 11 | TABLE ACCESS STORAGE FULL| XLA_AE_LINES | 6645K| 69M | 467 (10)| 00:00:01 | 1 | 610 | Q1,00 | PCWP | |

————————————————————————————————————————————————-

Note

—–

– Degree of Parallelism is 8 because of hint

#######################

Smart Scan Stats

#######################

NAME VALUE

————————————————- ———-

cell physical IO interconnect bytes 93,569,024

cell flash cache read hits 1,053

CPU used by this session 283

#######################

Smart Scan Efficiency *

#######################

no rows selected

#######################

Waits

#######################

STAT TIME_WAITED TOTAL_WAITS

———————————– —————- —————-

events in waitclass Other 352 70

SQL*Net message from client 107 26

PX Deq: Execute Reply 55 640

cell single block physical read 29 589

row cache lock 2 706

gc cr grant 2-way 2 237

PX Deq: Parse Reply 2 16

cell multiblock physical read 1 5

PX Deq: Join ACK 1 16

Summary from above:

– The query again did a parallel, full-scan on XLA_AE_LINES with a PARTITION RANGE ALL operation

– The query did NOT benefit from smart scan, even though it should have done a parallel direct read

– … but wait – looking at the wait event information, we see no evidence of “direct path read” waits, and without these we can’t get a smart scan

– We have no “cell smart%” waits for the session

This is puzzling – according to a couple of sources, certainly direct reads should have happened and these should have been offloaded. In my environment, it doesn’t look like in-memory parallel execution because all the requisite initialization parameters are defaulted:

1 select a.ksppinm name, b.ksppstvl value, b.ksppstdf isdefault

2 from x$ksppi a, x$ksppsv b

3 where a.indx = b.indx

4 and a.ksppinm in (‘parallel_degree_policy’,’_parallel_statement_queuing’,

5 ‘_parallel_cluster_cache_policy’)

6 –and b.ksppstdf=’FALSE’

7* order by 1,2

SYS @ visx1> /

_parallel_cluster_cache_policy ADAPTIVE TRUE

_parallel_statement_queuing FALSE FALSE

parallel_degree_policy MANUAL FALSE

So my working theory is that the size of the actual table isn’t small enough for Oracle to think it’s better to do parallel direct reads on. I’ll show some examples later on that attempt to prove this out.

Smart Scan Test Case #4: Full-scan XLA_AE_LINES, Parallel Query, Large Table

Now I’m going to simply create a larger set of tables to test with. At this point, we assume Oracle can choose direct path reads when the size of the segment being scanned is roughly 5 times “_small_table_threshold”, which is defined as 2% the size of the buffer cache. So let’s try to estimate how large we need our table to be:

APPS @ visx1> select a.ksppinm name, b.ksppstvl value, b.ksppstdf isdefault

2 from x$ksppi a, x$ksppsv b

3 where a.indx = b.indx

4 and 1=1

5 and a.ksppinm like ‘%’||’&parm’||’%’

6 –and b.ksppstdf=’FALSE’

7 order by 1,2

8 /

Enter value for parm: small_table

old 5: and a.ksppinm like ‘%’||’&parm’||’%’

new 5: and a.ksppinm like ‘%’||’small_table’||’%’

_small_table_threshold 27284 TRUE

APPS @ visx1> select 5 * 27284

2 from dual;

136420

1 row selected.

So it looks like we need a segment with more than 136,420 blocks. Let’s look at what our XLA_AE_LINES table shows:

SYS @ visx1> select blocks from dba_tables

2 where table_name=’XLA_AE_LINES’;

BLOCKS

———-

11225

1 row selected.

SYS @ visx1>

SYS @ visx1> select sum(blocks) from dba_tab_partitions

2 where table_name=’XLA_AE_LINES’;

SUM(BLOCKS)

———–

11225

1 row selected.

SYS @ visx1>

SYS @ visx1> select sum(blocks) from dba_tab_subpartitions

2 where table_name=’XLA_AE_LINES’;

SUM(BLOCKS)

———–

11225

1 row selected.

Based on the above, we’ve got 11,225 blocks in our partitioned table – about 10% of the size we need. The number of rows in XLA_AE_LINES is:

APPS @ visx1> select count(*) from xla_ae_lines;

COUNT(*)

———-

6638073

So we need to create a table with about 70 million rows in order to test this theory out. I’m going to create two tables – one a partitioned table with the same partitioning scheme as the original, and one un-partitioned. Excerpts from the code are below:

652 subpartition psb_2011 values less than (to_date(’01-JAN-2012′,’dd-MON-yyyy’)),

653 subpartition psb_max values less than (maxvalue)

654 ), partition partother values (default))

655 tablespace apps_ts_tx_data nologging

656 as

657 select * from xla.xla_ae_lines

658 /

Table created.

Elapsed: 00:01:01.41

SQL>

1 create table xla.xla_ae_lines_unpart

2 nologging

3* as select * from xla_ae_lines

SQL> /

Table created.

Elapsed: 00:00:21.74

SQL> select count(*) from xla.xla_ae_lines_unpart;

COUNT(*)

———-

6638073

Elapsed: 00:00:00.94

SQL> select count(*) from xla.xla_ae_lines_part;

COUNT(*)

———-

6638073

Now we’re going to load it up with data, up to over 100 million rows – I’ll spare the details, it wasn’t very scientific:

SQL> select count(*) from xla.xla_ae_lines_part;

COUNT(*)

———-

106209168

Elapsed: 00:00:10.70

SQL> select count(*) from xla.xla_ae_lines_unpart;

COUNT(*)

———-

106209168

Ok, now let’s run a parallel query against both tables and see if we can see smart scan. I’ll start with the partitioned table:

#######################

Test Query

#######################

APPS @ visx1> @exq.sql

APPS @ visx1> select count(*) from

2 (select /*+ parallel (8) */

3 code_combination_id,

4 sum(entered_dr), sum(entered_cr)

5 from xla.xla_ae_lines_part

6 group by code_combination_id

7 );

COUNT(*)

———-

5573

APPS @ visx1> set echo off

(dbms_xplan.display_cursor(‘&_sqlid’,&_cnum,’TYPICAL’))

new 1: select * from table (dbms_xplan.display_cursor(‘a7zk1u1w63w00′, 0,’TYPICAL’))

PLAN_TABLE_OUTPUT

——————————————————————————————————————————————————————————————————–

SQL_ID a7zk1u1w63w00, child number 0

————————————-

select count(*) from (select /*+ parallel (8) */ code_combination_id,

sum(entered_dr), sum(entered_cr) from xla.xla_ae_lines_part group by

code_combination_id )

Plan hash value: 2859844103

——————————————————————————————————————————————————

——————————————————————————————————————————————————

| 0 | SELECT STATEMENT | | | | 9070 (100)| | | | | | |

| 1 | SORT AGGREGATE | | 1 | | | | | | | | |

| 2 | PX COORDINATOR | | | | | | | | | | |

| 3 | PX SEND QC (RANDOM) | :TQ10001 | 1 | | | | | | Q1,01 | P->S | QC (RAND) |

| 4 | SORT AGGREGATE | | 1 | | | | | | Q1,01 | PCWP | |

| 5 | VIEW | | 5573 | | 9070 (30)| 00:00:01 | | | Q1,01 | PCWP | |

| 6 | HASH GROUP BY | | 5573 | 61303 | 9070 (30)| 00:00:01 | | | Q1,01 | PCWP | |

| 7 | PX RECEIVE | | 5573 | 61303 | 9070 (30)| 00:00:01 | | | Q1,01 | PCWP | |

| 8 | PX SEND HASH | :TQ10000 | 5573 | 61303 | 9070 (30)| 00:00:01 | | | Q1,00 | P->P | HASH |

| 9 | HASH GROUP BY | | 5573 | 61303 | 9070 (30)| 00:00:01 | | | Q1,00 | PCWP | |

| 10 | PX PARTITION RANGE ALL | | 106M| 1114M| 7073 (10)| 00:00:01 | 1 | LAST | Q1,00 | PCWC | |

| 11 | TABLE ACCESS STORAGE FULL| XLA_AE_LINES_PART | 106M| 1114M| 7073 (10)| 00:00:01 | 1 | 610 | Q1,00 | PCWP | |

——————————————————————————————————————————————————

Note

—–

– Degree of Parallelism is 8 because of hint

#######################

Smart Scan Stats

#######################

NAME VALUE

———————————————————————- ——————–

cell physical IO bytes eligible for predicate offload 1,366,065,152

cell physical IO interconnect bytes 1,088,400,208

cell physical IO interconnect bytes returned by smart scan 1,082,813,264

physical reads 167,438

physical reads direct 166,756

CPU used by this session 2,269

cell flash cache read hits 647

#######################

Smart Scan Efficiency *

#######################

Eligible MB SS Returned MB Pct Saved

——————– ——————– ———

1,303 1,038 20.33

#######################

Waits

#######################

STAT TIME_WAITED TOTAL_WAITS

———————————– ———————-

PX Deq: Execute Reply 497 645

SQL*Net message from client 73 27

events in waitclass Other 50 61

cell single block physical read 36 567

cursor: pin S wait on X 2 1

PX Deq: Parse Reply 2 16

gc cr grant 2-way 2 216

row cache lock 2 578

cell multiblock physical read 1 5

cell list of blocks physical read 1 1

Next, we’ll do it on our un-partitioned table:

#######################

Test Query

#######################

APPS @ visx1> @exq.sql

APPS @ visx1> select count(*) from

2 (select /*+ parallel (8) */

3 code_combination_id,

4 sum(entered_dr), sum(entered_cr)

5 from xla.xla_ae_lines_unpart

6 group by code_combination_id

7 );

COUNT(*)

———-

5573

APPS @ visx1> set echo off

old 1: select * from table (dbms_xplan.display_cursor(‘&_sqlid’,&_cnum,’TYPICAL’))

new 1: select * from table (dbms_xplan.display_cursor(‘8k1wkzavyambg’, 0,’TYPICAL’))

PLAN_TABLE_OUTPUT

SQL_ID 8k1wkzavyambg, child number 0

————————————-

select count(*) from (select /*+ parallel (8) */ code_combination_id,

sum(entered_dr), sum(entered_cr) from xla.xla_ae_lines_unpart group by

code_combination_id )

Plan hash value: 1164237767

—————————————————————————————————————————————-

—————————————————————————————————————————————-

| 0 | SELECT STATEMENT | | | | 108K(100)| | | | |

| 1 | SORT AGGREGATE | | 1 | | | | | | |

| 2 | PX COORDINATOR | | | | | | | | |

| 3 | PX SEND QC (RANDOM) | :TQ10001 | 1 | | | | Q1,01 | P->S | QC (RAND) |

| 4 | SORT AGGREGATE | | 1 | | | | Q1,01 | PCWP | |

| 5 | VIEW | | 5573 | | 108K (3)| 00:00:05 | Q1,01 | PCWP | |

| 6 | HASH GROUP BY | | 5573 | 61303 | 108K (3)| 00:00:05 | Q1,01 | PCWP | |

| 7 | PX RECEIVE | | 5573 | 61303 | 108K (3)| 00:00:05 | Q1,01 | PCWP | |

| 8 | PX SEND HASH | :TQ10000 | 5573 | 61303 | 108K (3)| 00:00:05 | Q1,00 | P->P | HASH |

| 9 | HASH GROUP BY | | 5573 | 61303 | 108K (3)| 00:00:05 | Q1,00 | PCWP | |

| 10 | PX BLOCK ITERATOR | | 106M| 1114M| 106K (1)| 00:00:05 | Q1,00 | PCWC | |

|* 11 | TABLE ACCESS STORAGE FULL| XLA_AE_LINES_UNPART | 106M| 1114M| 106K (1)| 00:00:05 | Q1,00 | PCWP | |

—————————————————————————————————————————————-

Predicate Information (identified by operation id):

—————————————————

11 – storage(:Z>=:Z AND :Z<=:Z)

Note

—–

– Degree of Parallelism is 8 because of hint

#######################

Smart Scan Stats

#######################

NAME VALUE

———————————————————————- ——————–

cell physical IO bytes eligible for predicate offload 23,030,530,048

cell physical IO interconnect bytes 1,873,421,824

cell physical IO interconnect bytes returned by smart scan 1,867,032,064

physical reads 2,812,124

physical reads direct 2,811,344

CPU used by this session 2,482

cell flash cache read hits 749

#######################

Smart Scan Efficiency *

#######################

Eligible MB SS Returned MB Pct Saved

——————– ——————– ———

21,964 1,787 91.87

1 row selected.

#######################

Waits

#######################

STAT TIME_WAITED TOTAL_WAITS

———————————– —————- —————-

PX Deq: Execute Reply 579 123

SQL*Net message from client 313 27

events in waitclass Other 43 57

cell single block physical read 38 751

row cache lock 2 299

gc cr grant 2-way 2 187

PX Deq: Parse Reply 1 16

cell multiblock physical read 1 5

So what did we learn from these tests?

– Both queries benefited from smart scan

– Both queries did direct reads

– We didn’t see “cell smart%” waits

– The size of the segment/granule being requested does play a role in where direct read is done, which governs smart scan

– There was a big difference in “cell physical IO bytes eligible for predicate offload” between the partitioned and unpartitioned table – in the case of the former, it was basically the number of bytes in the partitions, not summed, and in the latter it was the bytes in the table

Summary I

– In order for Exadata to do smart scans, direct reads are required

– Direct reads can either be serial direct reads of parallel direct reads

– In both cases, Oracle determines this based on the amount of data being requested. In other words, the size of the table (blocks), the size of the partition, or the size of the PQ granule

– This volume of data is influenced by “_small_table_threshold” multiplied by 5, or so we’re lead to believe so far. We’ll prove this in the next section

– The decision to do smart scans is made on the storage cell(s). As Oracle sends iDB messages to the cell, the message includes metadata that tells the cell what data to retrieve and a key characteristic of whether it’s being requested via direct path mechanism. If so, the cells act as column/row servers, and if not, they revert (or downgrade) to block servers

A Quick Word about Offload Efficiency

In the previous tests, I’ve shown some “offload efficiency” statistics. In my calculations, it’s the ratio of bytes sent over the InfiniBand interconnect to bytes eligible for predicate offload, although I’ve seen other formulas used by other folks. The point is this – it’s supposed to be an indication of how much data is saved via smart scan. But there are problems with this approach. First, as calculated, its scope is per SQL_ID – I’m grabbing the data from V$SQL. If you do a query that does a massive sort that has “direct path write” activities in it, data will be sent from the compute node *to* the storage cells, and then potentially doubled or tripled depending on ASM redundancy settings. This alone can drive the calculation negative, since in my definition it looks like this:

select

IO_CELL_OFFLOAD_ELIGIBLE_BYTES/1024/1024 val1,

IO_INTERCONNECT_BYTES/10241/024 val2,

round(100*(1-(IO_INTERCONNECT_BYTES/IO_CELL_OFFLOAD_ELIGIBLE_BYTES)),2) efficiency

from v$sql

where sql_id=’&_sqlid’

Second, it’s an efficiency, and although it does give an indication of how much data you saved in transit from cell to compute node, it’s still an “efficiency” number as I wrote it.

Direct Read Test Cases

As direct reads are a key to unlocking smart scan, it’s important to appreciate how Oracle determines whether to perform direct reads. As the previous examples showed, the size (in blocks, granules, etc) of the full segment scan dictates when direct read mechanism is used, and Oracle documents that this is based on 5 times “_small_table_threshold”, which is set to 2% the size of the buffer cache. We want to prove when direct reads kick in, and what sort of impact adaptive direct reads happen on Oracle’s choice of direct reads and ultimately, smart scan. In this section, I’m going to decrease our buffer cache size from 10GB to 4GB to make the test cases easier to demonstrate without forcing direct reads.

First, let’s show our buffer cache size and current value for small table threshold:

APPS @ visx1> show sga

Total System Global Area 8551575552 bytes

Fixed Size 2245480 bytes

Variable Size 4412411032 bytes

Database Buffers 3992977408 bytes

Redo Buffers 143941632 bytes

APPS @ visx1> @sysparm

APPS @ visx1> select a.ksppinm name, b.ksppstvl value, b.ksppstdf isdefault

2 from x$ksppi a, x$ksppsv b

3 where a.indx = b.indx

4 and 1=1

5 and a.ksppinm like ‘%’||’&parm’||’%’

6 –and b.ksppstdf=’FALSE’

7 order by 1,2

8 /

Enter value for parm: small_table

old 5: and a.ksppinm like ‘%’||’&parm’||’%’

new 5: and a.ksppinm like ‘%’||’small_table’||’%’

_small_table_threshold 9062 TRUE

So for now, our small table threshold is 9062, which means that about direct reads should commence when our blocks reaches 45310. We’re going to do a couple of tests to prove this, one in a tablespace with ASSM and one with MSSM.

I’m also going to show our non-standard, “interesting” initialization parameters, for reference:

APPS @ visx1> select a.ksppinm name, b.ksppstvl value, b.ksppstdf isdefault

2 from x$ksppi a, x$ksppsv b

3 where a.indx = b.indx

4 and a.ksppinm like ‘%’||’&parm’||’%’

5 and b.ksppstdf=’FALSE’

6 order by 1,2

7 /

__db_cache_size 3976200192 FALSE

__java_pool_size 16777216 FALSE

__large_pool_size 117440512 FALSE

__pga_aggregate_target 8589934592 FALSE

__sga_target 8589934592 FALSE

__shared_io_pool_size 0 FALSE

__shared_pool_size 4294967296 FALSE

__streams_pool_size 0 FALSE

_file_size_increase_increment 143289344 FALSE

_kill_diagnostics_timeout 140 FALSE

_lm_rcvr_hang_allow_time 140 FALSE

_optimizer_ignore_hints FALSE FALSE

_parallel_statement_queuing FALSE FALSE

_serial_direct_read AUTO FALSE

cluster_database TRUE FALSE

cluster_interconnects 192.168.10.1 FALSE

compatible 11.2.0.3.0 FALSE

db_block_size 8192 FALSE

filesystemio_options SETALL FALSE

instance_number 1 FALSE

open_cursors 1000 FALSE

parallel_adaptive_multi_user FALSE FALSE

parallel_degree_policy MANUAL FALSE

parallel_execution_message_size 16384 FALSE

parallel_max_servers 128 FALSE

parallel_min_servers 32 FALSE

pga_aggregate_target 8589934592 FALSE

pre_page_sga FALSE FALSE

processes 1024 FALSE

recyclebin OFF FALSE

sessions 1560 FALSE

sga_max_size 8589934592 FALSE

sga_target 8589934592 FALSE

shared_pool_size 4294967296 FALSE

sql92_security TRUE FALSE

Test Functions, Tables, and Intro Stuff

To do our tests, I’m going to heavily reference some work that that Alex Fatkulin published in his blog here – http://afatkulin.blogspot.com/2009/01/11g-adaptive-direct-path-reads-wha….

I’ll modify the function he’s built, but first, I’m going to create a MSSM tablespace and an ASSM tablespace, then create tables inside each of them to do our testing with:

SYS @ visx1> create tablespace dr_mssm

2 datafile ‘+DATA_CM01’ size 64m segment space management manual

3 /

Tablespace created.

Elapsed: 00:00:00.52

SYS @ visx1

1 create tablespace dr_assm

2* datafile ‘+DATA_CM01’ size 64m segment space management auto

SYS @ visx1>

SYS @ visx1> /

Tablespace created.

Elapsed: 00:00:00.35

Now I’ll create two tables, one in each tablespace:

APPS @ visx1> create table t_mssm

2 (col1 varchar2(100))

3 pctused 1

4 pctfree 99

5 tablespace dr_mssm;

Table created.

1 create table t_assm

2 (col1 varchar2(100))

3* tablespace dr_assm

APPS @ visx1> /

Table created..02

APPS @ visx1>

The function I’ll use looks like this:

APPS @ visx1> create or replace function calc_dr_mssm return number is

2 l_prd number;

3 l_blocks number:=0;

4 l_cnt number:=0;

5 begin

6 execute immediate ‘truncate table t_mssm’;

7 loop

8 insert /*+ append */ into t_mssm

9 select rpad(‘*’, 100, ‘*’)

10 from dual;

11 commit;

12 l_blocks:=l_blocks + 1;

13 execute immediate ‘alter system flush buffer_cache’;

14 select /*+ full(t) */ count(*) into l_cnt from t_mssm;

15 select value into l_prd

16 from v$segment_statistics

17 where owner=user

18 and object_name=’T_MSSM’

19 and statistic_name=’physical reads direct’;

20 exit when l_prd > 0;

21 end loop;

22 return l_blocks;

23 end;

24 /

Function created.

The function CALC_DR_ASSM looks similar, only the table names have changed.

Direct Read Threshold, T_MSSM

Now we’ll call CALC_DR_MSSM. Remember, our expected threshold is 45,310 blocks:

APPS @ visx1> declare

2 l_th number;

3 begin

4 l_th:=calc_dr_mssm;

5 dbms_output.put_line(‘Started doing direct reads at: ‘||l_th||’ blocks’);

6 end;

7 /

Started doing direct reads at: 9063 blocks

PL/SQL procedure successfully completed.

It appears to have stopped right at the small table threshold, not small table threshold times 5. If we check V$SQL and DBA_SEGMENTS, we can see this:

APPS @ visx1> select sql_id,io_cell_offload_eligible_bytes

2 from v$sql

3 where sql_id=’960bzw5qrmp86′;

SQL_ID IO_CELL_OFFLOAD_ELIGIBLE_BYTES

————- ——————————

960bzw5qrmp86 74244096

1 row selected.

Elapsed: 00:00:00.01

APPS @ visx1> select bytes, blocks

2 from dba_segments

3 where segment_name=’T_MSSM’;

BYTES BLOCKS

———- ———-

75497472 9216

So this test least seems to contradict the small table threshold times 5 theory.

The test above was on 11.2.0.3 – now let’s test with a smaller buffer cache on 11.2.0.2:

SQL> show sga

Total System Global Area 1068937216 bytes

Fixed Size 2233336 bytes

Variable Size 704646152 bytes

Database Buffers 348127232 bytes

Redo Buffers 13930496 bytes

SQL> @sysparm

SQL> select a.ksppinm name, b.ksppstvl value, b.ksppstdf isdefault

2 from x$ksppi a, x$ksppsv b

3 where a.indx = b.indx

4 and 1=1

5 and a.ksppinm like ‘%’||’&parm’||’%’

6 –and b.ksppstdf=’FALSE’

7 order by 1,2

8 /

Enter value for parm: small_table

old 5: and a.ksppinm like ‘%’||’&parm’||’%’

new 5: and a.ksppinm like ‘%’||’small_table’||’%’

So our target is, or should be, 1099 blocks or 1099 * 5. Let’s test it:

APPS @ prod> declare

2 l_th number;

3 begin

4 l_th:=calc_dr_mssm;

5 dbms_output.put_line(‘Started doing direct reads at: ‘||l_th||’ blocks’);

6 end;

7 /

Started doing direct reads at: 1100 blocks

PL/SQL procedure successfully completed.

Again, this confirmed that direct reads started right around, “_small_table_threshold”, not the threshold times 5.

ASSM Tablespace Direct Read Threshold, Direct Path Inserts

In this test, we’ll run CALC_DR_ASSM passing the same parameters. In our function, we’ll still do direct path inserts, one row at a time:

APPS @ visx1> create or replace function calc_dr_assm return number is

2 l_prd number;

3 l_blocks number:=0;

4 l_cnt number:=0;

5 begin

6 execute immediate ‘truncate table t_assm’;

7 loop

8 insert /*+ append */ into t_assm

9 select rpad(‘*’, 100, ‘*’)

10 from dual;

11 commit;

12 l_blocks:=l_blocks + 1;

13 execute immediate ‘alter system flush buffer_cache’;

14 select /*+ full(t) */ count(*) into l_cnt from t_assm;

15 select value into l_prd

16 from v$segment_statistics

17 where owner=user

18 and object_name=’T_ASSM’

19 and statistic_name=’physical reads direct’;

20 exit when l_prd > 0;

21 end loop;

22 return l_blocks;

23 end;

24 /

Function created.

Also recall that our small table threshold is 9062 blocks. Here’s what our test output shows:

APPS @ visx1> declare

2 l_th number;

3 begin

4 l_th:=calc_dr_assm;

5 dbms_output.put_line(‘Started doing direct reads at: ‘||l_th||’ inserts’);

6 end;

7 /

Started doing direct reads at: 8924 inserts

As we can see above, direct reads started after 8,924 inserts. Let’s check the block count:

APPS @ visx1> select blocks

2 from dba_segments

3 where segment_name=’T_ASSM’;

BLOCKS

———-

9216

1 row selected.

Elapsed: 00:00:02.54

APPS @ visx1> select count(*) from t_assm;

COUNT(*)

———-

8924

So with only 8924 rows inserted, we’ve actually got them spread across 9216 blocks, which is relatively close to “_small_table_threshold”. This test demonstrates two things:

– With direct path inserts, Oracle allocates new blocks above the highwater mark, so our block counts are disproportionally high with respect to the number of rows – in spite of ASSM tablespace storage

– Direct path reads began after 9215 blocks, which is close to our small table threshold – not 5 times this. This validated our previous test case

ASSM Tablespace Direct Read Threshold, “Normal” Inserts

In this test, we’re going to do conventional inserts against a table in an ASSM tablespace, which should allow Oracle to more tightly pack blocks. We should see more rows inserted and more rows-per-block. And of course, we also want to show when direct path reads start. I’ll modify the function not only to do conventional inserts, but also to batch them up in larger chunks in efforts to get the program to run faster.

APPS @ visx1> create or replace function calc_dr_assm return number is

2 l_prd number;

3 l_blocks number:=0;

4 l_cnt number:=0;

5 begin

6 execute immediate ‘truncate table t_assm’;

7 loop

8 insert into t_assm

9 select rpad(‘*’, 100, ‘*’)

10 from dual connect by level<=500;

11 commit;

12 l_blocks:=l_blocks + 1;

13 execute immediate ‘alter system flush buffer_cache’;

14 select /*+ full(t) */ count(*) into l_cnt from t_assm;

15 select value into l_prd

16 from v$segment_statistics

17 where owner=user

18 and object_name=’T_ASSM’

19 and statistic_name=’physical reads direct’;

20 exit when l_prd > 0;

21 end loop;

22 return l_blocks;

23 end;

24 /

Function created.

Now for our test:

APPS @ visx1> declare

2 l_th number;

3 begin

4 l_th:=calc_dr_assm;

5 dbms_output.put_line(‘Started doing direct reads at: ‘||l_th||’ inserts’);

6 end;

7 /

Started doing direct reads at: 1096 inserts

This shows that after 1096 1000-row inserts, we began doing direct path reads. Let’s examine more closely:

APPS @ visx1> exec dbms_stats.gather_table_stats(‘APPS’,’T_ASSM’);

APPS @ visx1> l

1 select num_rows,blocks,num_rows/blocks

2 from dba_tables

3* where table_name=’T_ASSM’

APPS @ visx1> /

548000 9077 60.3723697

We’ve got 9077 blocks and about 60 rows per block. 9077 blocks is 15 blocks over the small table threshold, which corresponds to about 900 rows. Since we were doing row inserts in increments of 1000, the last insert pushed the block count above the threshold.

Again, this test showed that direct reads started at “_small_table_threshold”.

Cached Block Threshold: MSSM

According to Oracle’s documentation and Alex’s blog, direct reads stop happening when a specific threshold of the table’s blocks are already in cache. In this section, we’ll test this using our MSSM table. Let’s first check how many rows and blocks we have in our tables.

APPS @ visx1> select table_name,num_rows,blocks,num_rows/blocks

2 from dba_tables

3 where table_name in (‘T_MSSM’,’T_ASSM’);

T_MSSM 9063 9063 1

T_ASSM 548000 9077 60.3723697

We’ll create indexes on both tables so we can get an index scan/rowid lookup in this test case and the next one:

APPS @ visx1> create index i_t_mssm

2 on t_mssm (1);

Index created.

Elapsed: 00:00:00.11

APPS @ visx1> create index i_t_assm

2 on t_assm (1);

Index created.

The pseudo-code of our test plan will be to:

1. Define a cursor on the table

2. Open the cursor and fetch one row at a time – this should do an index scan and rowid lookup. In the case of our MSSM table, T_MSSM, one row will equate to one table block

3. After each row is fetched, we’ll run a full-scan on the table and check the physical direct read statistic

4. If it matches the previous one, it means a direct read wasn’t done and we stop

5. If it’s higher than the previous one, we’ll fetch another row and do another full-scan

Our code looks like this:

APPS @ visx1> create or replace function calc_cached_mssm

2 return number is

3 cursor l_cur is select /*+ index(t_mssm i_t_mssm) */ * from t_mssm;

4 l_v varchar2(100);

5 l_execs number:=0;

6 l_cnt number:=0;

7 l_prv number:=0;

8 begin

9 execute immediate ‘alter system flush buffer_cache’;

10 open l_cur;

11 loop

12 fetch l_cur into l_v;

13 l_execs:=l_execs+1;

14 select /*+ full(t) */ count(*) into l_cnt from t_mssm t;

15 select value into l_cnt

16 from v$segment_statistics

17 where owner=user

18 and object_name=’T_MSSM’

19 and statistic_name=’physical reads direct’;

20 exit when l_cnt = l_prv or l_cur%notfound;

21 l_prv:=l_cnt;

22 end loop;

23 close l_cur;

24 return l_execs;

25 end;

26 /

Function created.

If we run it, we see it stopping at 8772 blocks, or more accurately, after 8772 fetches of our cursor:

APPS @ visx1> declare

2 l_th number;

3 begin

4 l_th:=calc_cached_mssm;

5 dbms_output.put_line(‘Stopped doing direct reads at: ‘||l_th||’. ‘);

7 end;

8 /

Stopped doing direct reads at: 8772.

T_MSSM has 9216 blocks in it, so we basically stopped doing direct reads after (8772/9216) = 95% of the blocks existed in cache. This is quite a bit higher than in Alex’s blog, where he witnessed the cached block threshold at about 50% the number of blocks in the table.

Cached Block Threshold: ASSM

Similar to the above example, we’ll create a procedure to test T_ASSM, our table in an ASSM tablespace. This table has about 60 rows/block and 548,000 rows, so we’d expect to see it stop at about 520,600 fetches.

The test results look like this:

APPS @ visx1> declare

2 l_th number;

3 begin

4 l_th:=calc_cached_assm;

5 dbms_output.put_line(‘Stopped doing direct reads at: ‘||l_th||’. ‘);

7 end;

8 /

Stopped doing direct reads at: 548001.

Since our table has 548,000 rows, it seems from the above test that the full scan never reverted from direct reads, which may indicate that if the block counts are high, cached blocks has no impact on direct reads. Let’s validate that indeed our cursor handling was populating blocks in cache. I’m going to run a trace against the same type of program, minus the full-scans:

SQL> create or replace function calc_cached_assm_t2

2 return number is

3 cursor l_cur is select /*+ index(t_assm i_t_assm) */ * from t_assm;

4 l_v varchar2(100);

5 l_execs number:=0;

6 l_cnt number:=0;

7 l_prv number:=0;

8 begin

9 execute immediate ‘alter system flush buffer_cache’;

10 open l_cur;

11 loop

12 fetch l_cur into l_v;

13 l_execs:=l_execs+1;

14 exit when l_cur%notfound;

15 end loop;

16 close l_cur;

17 return l_execs;

18 end;

19 /

SQL> set serveroutput on size 20000

SQL> alter session set events ‘10046 trace name context forever, level 12’;

Session altered.

SQL> set echo on

SQL> declare

2 l_th number;

3 begin

4 l_th:=calc_cached_assm_t2;

5 dbms_output.put_line(‘Finished reads at: ‘||l_th||’. ‘);

6 end;

7 /

Finished reads at: 548001.

PL/SQL procedure successfully completed.

SQL> alter session set events ‘10046 trace name context off’;

Session altered.

The trace looks output like this:

SELECT /*+ index(t_assm i_t_assm) */ *

FROM

T_ASSM

call count cpu elapsed disk query current rows

——- —— ——– ———- ———- ———- ———- ———-

Parse 1 0.00 0.00 0 0 0 0

Execute 1 0.00 0.00 0 0 0 0

Fetch 548001 3.14 7.30 9132 1096002 0 548000

——- —— ——– ———- ———- ———- ———- ———-

total 548003 3.14 7.31 9132 1096002 0 548000

Additionally, we can confirm that the table is nearly 100% buffered based on the below:

SQL> set autotrace on

SQL> select count(*) from t_assm;

COUNT(*)

———-

548000

Execution Plan

———————————————————-

Plan hash value: 2636023310

———————————————————————————-

———————————————————————————-

| 0 | SELECT STATEMENT | | 1 | 301 (3)| 00:00:01 |

| 1 | SORT AGGREGATE | | 1 | | |

| 2 | INDEX STORAGE FAST FULL SCAN| I_T_ASSM | 548K| 301 (3)| 00:00:01 |

———————————————————————————-

Statistics

———————————————————-

0 recursive calls

0 db block gets

1091 consistent gets

0 physical reads

0 redo size

527 bytes sent via SQL*Net to client

520 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

1 rows processed

SQL> set autotrace off

SQL> @../sqlmon/mywaits

STAT TIME_WAITED TOTAL_WAITS

———————————– —————- —————-

SQL*Net message from client 10,538 37

Disk file operations I/O 0 70

SQL*Net message to client 0 38

events in waitclass Other 0 2

This test confirms that for the most part, direct reads were never changed to buffered reads as a result of data being cached. My theory is that it’s due to the total row count, not block count – Oracle adaptively decides to do serial direct reads if the data set is large enough. I haven’t validate this theory yet though …

Dirty Read Threshold: MSSM

Direct reads require a segment level checkpoint – if you’ve ever seen a wait for “enq: KO – fast object checkpoint”, you’re probably seeing this phenomenon. In this test, we’re going to update rows in our T_MSSM table in various ranges, starting with 0 rows and incrementing by one, then following with an attempt at a full scan via direct reads. The goal of this test is to determine at which point direct reads are disabled due to dirty blocks being in cache. Here’s the function:

SQL> create or replace function calc_dirty_mssm

2 return number is

3 l_v varchar2(100);

4 l_execs number:=0;

5 l_cnt number:=0;

6 l_prv number:=0;

7 begin

8 execute immediate ‘alter system flush buffer_cache’;

9 loop

10 update t_mssm set col1=col1 where rownum < l_execs;

11 commit;

12 l_execs:=l_execs+1;

13 select /*+ full(t) */ count(*) into l_cnt from t_mssm t;

14 select value into l_cnt

15 from v$segment_statistics

16 where owner=user

17 and object_name=’T_MSSM’

18 and statistic_name=’physical reads direct’;

19 exit when l_cnt = l_prv or l_execs > 9063;

20 l_prv:=l_cnt;

21 end loop;

22 return l_execs;

23 end;

24 /

Function created.

In the function, I’m putting in a clause to exit the loop when the execution count exceeds the block count of the table (9063) in the event dirty reads never stop direct path reads.

Test results show the following:

SQL> @mssm_test3

SQL> set serveroutput on size 20000

SQL> set echo on

SQL> declare

2 l_th number;

3 begin

4 l_th:=calc_dirty_mssm;

5 dbms_output.put_line(‘Stopped doing direct reads at: ‘||l_th||’. ‘);

6 end;

7 /

Stopped doing direct reads at: 3888.

PL/SQL procedure successfully completed.

SQL>

As we can see, direct reads stopped at 3888 blocks, which is about 43% the number of blocks. Let’s look at our wait statistics:

SQL> select stat,sum(time_waited) time_waited,

2 sum(total_waits) total_waits

3 from

4 (select c.event stat

5 ,c.time_waited,c.total_waits,

6 b.sid,b.serial#,nvl(b.module,b.program) what

7 from v$session b, v$session_event c

8 where c.sid=b.sid

9 and b.sid=userenv(‘sid’))

10 group by stat

11 having sum(time_waited) > 0

12 order by 2 desc

13 /

STAT TIME_WAITED TOTAL_WAITS

—————————————- —————- —————-

SQL*Net message from client 15,914 26

enq: KO – fast object checkpoint 12,754 7,759

cell smart table scan 9,697 160,595

events in waitclass Other 535 3,909

log file switch (checkpoint incomplete) 88 21

cell single block physical read 45 232

cell multiblock physical read 22 86

log file switch completion 4 1

gc cr multi block request 4 104

log buffer space 2 3

Notice the high wait time for “en: KO – fast object checkpoint” – this is an indication that the direct reads are motivating the object checkpoint, something that can be a relatively nontrivial serialization or concurrency issue.

Dirty Read Threshold: ASSM

Similar to the previous test case, we’re now going to try the same thing on our ASSM table, T_ASSM. Our function looks slightly different – we’re going to run updates in batches of 100:

SQL> create or replace function calc_dirty_assm

2 return number is

3 l_v varchar2(100);

4 l_execs number:=0;

5 l_bs number:=100;

6 l_cnt number:=0;

7 l_prv number:=0;

8 begin

9 execute immediate ‘alter system flush buffer_cache’;

10 loop

11 update t_assm set col1=col1 where rownum < l_execs+l_bs;

12 commit;

13 l_execs:=l_execs+l_bs;

14 select /*+ full(t) */ count(*) into l_cnt from t_assm t;

15 select value into l_cnt

16 from v$segment_statistics

17 where owner=user

18 and object_name=’T_ASSM’

19 and statistic_name=’physical reads direct’;

20 exit when l_cnt = l_prv or l_execs > 548000;

21 l_prv:=l_cnt;

22 end loop;

23 return l_execs;

24 end;

25 /

Function created.

Now we’ll run our test against our ASSM table, T_ASSM:

SQL> declare

2 l_th number;

3 begin

4 l_th:=calc_dirty_assm;

5 dbms_output.put_line(‘Stopped doing direct reads at: ‘||l_th||’. ‘);

6 end;

7 /

Stopped doing direct reads at: 255600.

PL/SQL procedure successfully completed.

Based on the way the code is written, our threshold was somewhere between 255500 and 255600 rows, or about 4260 blocks. This is about 46% the size of the table.

Direct Read Threshold, Oracle EBS R12 APPS Table

In this test, we’re going to simulate the direct read threshold on an Oracle EBS table, ONT.OE_ORDER_LINES_ALL. We’re doing this to test the impact with a real-world row size, but in general the test is very similar to the test for direct reads for a table stored in an ASSM tablespace. To setup the test, we’ll create a dummy table called OEL_TEST:

APPS @ visx1> create table oel_test

2 tablespace apps_ts_tx_data

3 as select * from ont.oe_order_lines_all

4 where 1=2;

Table created.

Elapsed: 00:00:00.29

APPS @ visx1>

Now, we’ll create a similar function to load data and validate direct reads against this table. For our insert operations, I’m going to do a IAS from OE_ORDER_LINES_ALL for a single, indexed column:

APPS @ visx1> create or replace function calc_dr_apps return number is

2 l_prd number;

3 l_blocks number:=0;

4 l_cnt number:=0;

5 begin

6 execute immediate ‘truncate table oel_test’;

7 loop

8 insert into oel_test

9 select * from

10 (select * from oe_order_lines_all

11 where line_id=388282)

12 connect by level<=500;

13 commit;

14 l_blocks:=l_blocks + 1;

15 execute immediate ‘alter system flush buffer_cache’;

16 select /*+ full(t) */ count(*) into l_cnt from oel_test;

17 select value into l_prd

18 from v$segment_statistics

19 where owner=user

20 and object_name=’OEL_TEST’

21 and statistic_name=’physical reads direct’;

22 exit when l_prd > 0;

23 end loop;

24 return l_blocks;

25 end;

26 /

Function created.

Since we’ve done some updating in previous tests and Oracle’s adjusted the buffer cache size, our new “_small_table_threshold” is 8986 – close to the original, but a bit less.

The test results are below:

SQL> declare

2 l_th number;

3 begin

4 l_th:=calc_dr_apps;

5 dbms_output.put_line(‘Started doing direct reads at: ‘||l_th||’ inserts’);

7 end;

8 /

Started doing direct reads at: 180 inserts

PL/SQL procedure successfully completed.

SQL> exec dbms_stats.gather_table_stats(‘APPS’,’OEL_TEST’);

PL/SQL procedure successfully completed.

SQL> select blocks, num_rows, num_rows/blocks

2 from dba_tables

3 where table_name=’OEL_TEST’;

9012 90000 9.98668442

Based on the above results, we started doing direct path reads at 9012 blocks, slightly higher than the small table threshold value. Bear in mind, the extra insert at the end of the loop could have added as many as 500 additional rows, which equates to 50 additional blocks. If you consider this, the threshold for direct reads was again right around small table threshold.

Cached Block Threshold, APPS Table

To measure our cached block threshold, we’ll run a function similar to the ASSM test case a few sections ago. We’re first going to need to create the necessary index to motivate an index scan:

SQL> create index i_oel_test on oel_test (1);

Index created.

SQL> exec dbms_stats.gather_table_stats(‘APPS’,’OEL_TEST’);

PL/SQL procedure successfully completed.

Our function looks like this:

SQL> create or replace function calc_cached_apps

2 return number is

3 cursor l_cur is select /*+ index(oel_test i_oel_test) */ * from oel_test;

4 l_v oel_test%rowtype;

5 l_execs number:=0;

6 l_cnt number:=0;

7 l_cnt2 number:=0;

8 l_prv number:=0;

9 begin

10 execute immediate ‘alter system flush buffer_cache’;

11 open l_cur;

12 loop

13 fetch l_cur into l_v;

14 l_execs:=l_execs+1;

15 select /*+ full(t) */ count(*) into l_cnt from oel_test t;

16 select value into l_cnt

17 from v$segment_statistics

18 where owner=user

19 and object_name=’OEL_TEST’

20 and statistic_name=’physical reads direct’;

21 exit when l_cnt = l_prv or l_cur%notfound;

22 l_prv:=l_cnt;

23 end loop;

24 close l_cur;

25 return l_execs;

26 end;

27 /

Function created.

Let’s run our test:

APPS @ visx1> declare

2 l_th number;

3 begin

4 l_th:=calc_cached_assm;

5 dbms_output.put_line(‘Stopped doing direct reads at: ‘||l_th||’. ‘);

7 end;

8 /

Stopped doing direct reads at: 89421.

Our table has 90000 rows, and it looks like we stopped doing direct reads after 89421 of these were in cache. This is more than 99% the table size, and based on this test it looks like cache block thresholds don’t influence direct path reads.

Dirty Read Threshold, APPS Table

In this test, we’ll test the threshold at which dirty reads impact direct reads. Our function looks like this:

SQL> create or replace function calc_dirty_apps

2 return number is

3 l_v varchar2(100);

4 l_execs number:=0;

5 l_bs number:=100;

6 l_cnt number:=0;

7 l_prv number:=0;

8 begin

9 execute immediate ‘alter system flush buffer_cache’;

10 loop

11 update oel_test set line_number=line_number,

12 open_flag=open_flag

13 where rownum < l_execs+l_bs;

14 commit;

15 l_execs:=l_execs+l_bs;

16 select /*+ full(t) */ count(*) into l_cnt from oel_test t;

17 select value into l_cnt

18 from v$segment_statistics

19 where owner=user

20 and object_name=’OEL_TEST’

21 and statistic_name=’physical reads direct’;

22 exit when l_cnt = l_prv or l_execs > 90000;

23 l_prv:=l_cnt;

24 end loop;

25 return l_execs;

26 end;

27 /

Function created.

Test results are provided below:

SQL> declare

2 l_th number;

3 begin

4 l_th:=calc_dirty_apps;

5 dbms_output.put_line(‘Stopped doing direct reads at: ‘||l_th||’. ‘);

6 end;

7 /

Stopped doing direct reads at: 39800.

PL/SQL procedure successfully completed.

We can see that direct reads stopped between 39700 and 39800 rows. With about 10 rows per block, we’ve got 3980 blocks, or about 44% the number of blocks in the table.

Direct Read Threshold, T_MSSM: Test 2

In the tests above we had “_serial_direct_read”=AUTO, which should be the default. However, queries from X$KSPPSV showed that the value was not the default, and it was also in upper-case. Oracle allows you to alter the parameter and set it to AUTO, but we’re questioning whether this indeed produces the default behavior or something else.

Since our direct read threshold didn’t measure up to what we expected based on other publications, I’m going to unset it and do the MSSM test again and see if there’s a difference.

SQL> alter system reset “_serial_direct_read” sid=’visx1′;

System altered.

SQL> ed

Wrote file afiedt.buf

1* alter system reset “_serial_direct_read” sid=’visx2′

SQL> /

System altered.

SQL> Disconnected from Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 – 64bit Production

With the Partitioning, Real Application Clusters, Automatic Storage Management, OLAP,

Data Mining and Real Application Testing options

[[email protected] direct_reads]$ srvctl stop database -d visx

[[email protected] direct_reads]$ srvctl start database -d visx

[[email protected] direct_reads]$ sqlplus / as sysdba

SQL*Plus: Release 11.2.0.3.0 Production on Sat Mar 10 12:04:21 2012

Connected to:

Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 – 64bit Production

With the Partitioning, Real Application Clusters, Automatic Storage Management, OLAP,

Data Mining and Real Application Testing options

SQL> @../sqlmon/sysparm

SQL> select a.ksppinm name, b.ksppstvl value, b.ksppstdf isdefault

2 from x$ksppi a, x$ksppsv b

3 where a.indx = b.indx

4 and a.ksppinm like ‘%’||’&parm’||’%’

5 and b.ksppstdf=’FALSE’

6 order by 1,2

7 /

Enter value for parm: serial_dir

old 4: and a.ksppinm like ‘%’||’&parm’||’%’

new 4: and a.ksppinm like ‘%’||’serial_dir’||’%’

no rows selected

With “_serial_direct_read” set to its default (auto, not AUTO), we’re now going run our direct read threshold test again and see how it compares to the test in the “MSSM Tablespace Direct Read Threshold” test. Our current value for “_small_table_threshold” is 8949, as communicated above:

APPS @ visx1> declare

2 l_th number;

3 begin

4 l_th:=calc_dr_mssm;

5 dbms_output.put_line(‘Started doing direct reads at: ‘||l_th||’ blocks’);

6 end;

7 /

Started doing direct reads at: 8950 blocks

PL/SQL procedure successfully completed.

The above test validates that (8950-1) = 8949 = “_small_table_threshold”, which indicates that whether “_serial_direct_read”= AUTO or auto, Oracle still did direct reads at the “_small_table_threshold” setting.

Parallel Direct Read Test Case

In this test we’re going to test the threshold for when parallel direct reads are done for parallel queries. We’ll use our partitioned XLA_AE_LINES_PART table as our test case, similar to the tests against T_ASSM or OEL_TEST, but we’ll need to prepare a test temporary table to hold a sampling of data to make our program run reasonably fast. We’ll first build our temp table:

APPS @ visx1> create table xla_temp

2 as select * from xla_ae_lines

3 where 1=2;

Now we’ll do a simple script to insert one row into it from each subparition:

select ‘insert into xla_temp select * from xla_ae_lines subpartition (‘||subpartition_name||’) where rownum=1;’

from dba_tab_subpartitions

where table_name=’XLA_AE_LINES’

We’ll save this to a script and execute it. When done, XLA_TEMP looks like this:

APPS @ visx1> select count(*) from xla_temp;

1 row selected.

Elapsed: 00:00:00.00

APPS @ visx1> insert into xla_temp

2 select * from xla_temp;

67 rows created.

Elapsed: 00:00:00.01

APPS @ visx1> select count(*) from xla_temp;

134

1 row selected.

Elapsed: 00:00:00.00

APPS @ visx1> commit;

Now we’ve got a 134 row temporary table that we’ll use as the basis for inserting new rows into XLA_AE_LINES_PART. Let’s drop this table and re-create it first:

SQL> prompt Dropping existing partitioned table

Dropping existing partitioned table

SQL> drop table xla.xla_ae_lines_part;

Table dropped.

Elapsed: 00:00:00.45

SQL> prompt Creating partitioned table

Creating partitioned table

SQL>

SQL> CREATE TABLE XLA.XLA_AE_LINES_PART

2 PARTITION BY list (application_id)

3 subpartition by range (accounting_date)

4 (partition AP values (200)

5 (subpartition ap_1995 values less than (to_date(’01-JAN-1996′,’dd-MON-yyyy’)) compress for query high,

6 subpartition ap_1996 values less than (to_date(’01-JAN-1997′,’dd-MON-yyyy’)) compress for query high,

<< lines deleted >>

653 subpartition psb_max values less than (maxvalue)

654 ), partition partother values (default))

655 tablespace apps_ts_tx_data nologging

656 as

657 select * from xla.xla_ae_lines

658 where 1=2

659 /

Table created.

Next, we’ll create a function to test parallel direct reads:

APPS @ visx1> create or replace function calc_pdr_xla return number is

2 l_prd number;

3 l_blocks number:=0;

4 l_cnt number:=0;

5 begin

6 execute immediate ‘truncate table xla.xla_ae_lines_part’;

7 loop

8 insert into xla.xla_ae_lines_part

9 select * from xla_temp;

10 commit;

11 l_blocks:=l_blocks + 1;

12 execute immediate ‘alter system flush buffer_cache’;

13 select /*+ full(t) parallel (8) */ count(*) into l_cnt from xla.xla_ae_lines_part t;

14 select sum(value) into l_prd

15 from v$segment_statistics

16 where owner=’XLA’

17 and object_name=’XLA_AE_LINES_PART’

18 and statistic_name=’physical reads direct’;

19 exit when l_prd > 0;

20 end loop;

21 return l_blocks;

22 end;

23 /

Function created.

Finally, we’ll test the function. Results are below:

APPS @ visx1> declare

2 l_th number;

3 begin

4 l_th:=calc_pdr_xla;

5 dbms_output.put_line(‘Started doing direct reads at: ‘||l_th||’ inserts’);

6 end;

7 /

Started doing direct reads at: 3790 inserts

PL/SQL procedure successfully completed.

After 3790 134-row inserts, direct reads started. If we look at the block counts for the table, we see this:

APPS @ visx1> @b

SUM(BLOCKS)

———–

9616

APPS @ visx1> select count(*) from xla.xla_ae_lines_part;

COUNT(*)

———-

507860

The block count of 9616 is a bit over the value for small table threshold, but remember we’ve been batching inserts together so it’s reasonable to see a higher value of both blocks and row-count values. If we look at block counts per-partition, we see this:

1 select table_name,partition_name,blocks

2 from dba_tab_partitions

3* where table_name=’XLA_AE_LINES_PART’

APPS @ visx1> /

TABLE_NAME PARTITION_NAME BLOCKS

—————————— —————————— ———-

XLA_AE_LINES_PART AP 2363

XLA_AE_LINES_PART AR 933

XLA_AE_LINES_PART CE 0

XLA_AE_LINES_PART COREBANK 836

XLA_AE_LINES_PART CST 503

XLA_AE_LINES_PART DPP 0

XLA_AE_LINES_PART FUN 0

XLA_AE_LINES_PART FV 925

XLA_AE_LINES_PART GMF 134

XLA_AE_LINES_PART IGC 0

XLA_AE_LINES_PART IGI 0

XLA_AE_LINES_PART INSITBANK 73

XLA_AE_LINES_PART LNS 523

XLA_AE_LINES_PART OFA 972

XLA_AE_LINES_PART OKL 402

XLA_AE_LINES_PART OZF 0

XLA_AE_LINES_PART PA 770

XLA_AE_LINES_PART PARTOTHER 0

XLA_AE_LINES_PART PAY 0

XLA_AE_LINES_PART PN 0

XLA_AE_LINES_PART PO 568

XLA_AE_LINES_PART PSB 0

What does this tell us? It says that parallel direct reads started when the overall block count (i.e., total number of blocks across all partitions) is above “_small_table_threshold”. Let’s try a full-partition scan on one of the larger partitions, the “AP” partition. First, let’s get a current physical direct read count for all “AP” partitions:

APPS @ visx1> select sum(value )

2 from v$segment_statistics

3 where owner=’XLA’

4 and object_name=’XLA_AE_LINES_PART’

5 and subobject_name like ‘AP%’

6 and statistic_name=’physical reads direct’

7 /

SUM(VALUE)

———-

4726

Now we’ll full-scan the partition:

APPS @ visx1> select count(*) from xla.xla_ae_lines_part

2 partition (AP);

COUNT(*)

———-

98540

When done, we’ll re-run our query against V$SEGMENT_STATISTICS:

APPS @ visx1> select sum(value )

2 from v$segment_statistics

3 where owner=’XLA’

4 and object_name=’XLA_AE_LINES_PART’

5 and subobject_name like ‘AP%’

6 and statistic_name=’physical reads direct’

7 /

SUM(VALUE)

———-

4726

As you can see, we did no direct reads on the single partition, and this is because the number of blocks in the partition, 2363, is less than “_small_table_threshold”.

Oracle Apps Direct Read Test Case

In this section, I’m going to show some tests on a production Oracle EBS 12.1.2 environment running on Oracle 11.2.0.2. This environment is running on AIX, not Exadata. The purpose here is to determine whether Oracle switches to direct reads at the “_small_table_threshold” boundary in a real-life, production system. Here’s what our cache size and relevant direct read parameters are set to:

SQL> show sga

Total System Global Area 4275781632 bytes

Fixed Size 2226472 bytes

Variable Size 2650801880 bytes

Database Buffers 1610612736 bytes

Redo Buffers 12140544 bytes

SQL>

SQL> select a.ksppinm name, b.ksppstvl value, b.ksppstdf isdefault

2 from x$ksppi a, x$ksppsv b

3 where a.indx = b.indx

4 and a.ksppinm in (‘_small_table_threshold’,’_serial_direct_read’)

5 /

_small_table_threshold 3744 TRUE

_serial_direct_read auto TRUE

So we’ve got default values for both parameters. Below is a query that shows what we’d expect with direct reads, based on the assumption the actual threshold is 5 times “_small_table_threshold”:

SQL> select owner,table_name,blocks,

2 (case when blocks > (3744*5) then ‘Y’

3 else ‘N’

4 end) dr

5 from dba_tables

6 where blocks > 3200

7 and table_name not like ‘MLOG%’

8 and owner in

9 (select oracle_username from applsys.fnd_oracle_userid)

10 /

Owner Table Blocks Expect DR

————— ———————————– ——— ———-

APPLSYS AD_FILES 7425 N

APPLSYS AD_FILE_VERSIONS 13123 N

APPLSYS AD_PATCH_COMMON_ACTIONS 13633 N

AK AK_REGION_ITEMS 6969 N

APPLSYS AD_PATCH_RUN_BUGS 3379 N

APPLSYS AD_PATCH_RUN_BUG_ACTIONS 10446 N

APPLSYS AD_SNAPSHOT_BUGFIXES 13696 N

ALR ALR_ALERT_CHECKS 3394 N

APPLSYS AD_SNAPSHOT_FILES 51416 Y

APPLSYS AD_TASK_TIMING 7929 N

APPLSYS FND_COLUMNS 11339 N

APPLSYS FND_CONCURRENT_REQUESTS 6181 N

APPLSYS FND_ENV_CONTEXT 10271 N

AR HZ_HIERARCHY_NODES 10191 N

AR HZ_RELATIONSHIPS 51098 Y

APPLSYS FND_LOBS 10558 N

AX AX_RULE_LINES 9060 N

BEN BEN_ELIG_PER_ELCTBL_CHC 16549 N

BEN BEN_ELIG_PER_F 16055 N

BEN BEN_ELIG_PER_OPT_F 29616 Y

BEN BEN_ENRT_RT 12008 N

BEN BEN_PERSON_ACTIONS 7284 N

BEN BEN_PRTT_ENRT_RSLT_F 3835 N

BEN BEN_REPORTING 19051 Y

XXXX XX_AS400_IIMTM 3258 N

CZ CZ_IMP_PS_NODES 8358 N

AZ AZ_DIFF_RESULTS 721535 Y

AZ AZ_REPORTER_DATA 3351285 Y

ICX ICX_SESSION_ATTRIBUTES 4705 N

BOM BOM_STRUCTURES_B 40261 Y

BOM BOM_COMPONENTS_B 4072 N

BOM BOM_INVENTORY_COMPS_INTERFACE 5709 N

BOM CST_ITEM_COSTS 21648 Y

INV MTL_INTERFACE_ERRORS 3520 N

INV MTL_ITEM_CATEGORIES 73312 Y

INV MTL_ITEM_REVISIONS_B 31544 Y

INV MTL_ITEM_REVISIONS_INTERFACE 39273 Y

INV MTL_ITEM_REVISIONS_TL 14222 N

INV MTL_PENDING_ITEM_STATUS 17744 N

INV MTL_SYSTEM_ITEMS_B 218803 Y

INV MTL_SYSTEM_ITEMS_INTERFACE 253048 Y

INV MTL_SYSTEM_ITEMS_TL 26366 Y

APPLSYS FND_NEW_MESSAGES 5236 N

APPLSYS FND_VIEWS 7110 N

APPLSYS FND_VIEW_COLUMNS 5709 N

APPLSYS JDR_ATTRIBUTES 20008 Y

APPLSYS JDR_COMPONENTS 8008 N

APPLSYS WF_DEFERRED 6228 N

APPLSYS WF_ITEM_ACTIVITY_STATUSES 3237 N

APPLSYS WF_ITEM_ATTRIBUTE_VALUES 27975 Y

APPLSYS AD_PROGRAM_RUN_TASK_JOBS 14270 N

AR HZ_GEOGRAPHIES 4308 N

AR HZ_GEOGRAPHY_IDENTIFIERS 4749 N

EGO EGO_ITEM_TEXT_TL 23449 Y

ENI ENI_OLTP_ITEM_STAR 43289 Y

APPLSYS DR$FND_LOBS_CTX$I 214198 Y

APPLSYS WF_JAVA_DEFERRED 4056 N

ICX DR$ICX_CAT_ITEMSCTXDESC_HDRS$I 6323 N

ICX ICX_CAT_ITEMS_CTX_DTLS_TLP 22493 Y

ICX ICX_CAT_ITEMS_CTX_HDRS_TLP 4749 N

ICX ICX_CAT_ATTRIBUTE_VALUES 10813 N

ICX ICX_CAT_ATTRIBUTE_VALUES_TLP 7725 N

INV MTL_ITEM_BULKLOAD_RECS 38715 Y

PO PO_ATTRIBUTE_VALUES 10462 N

PO PO_ATTRIBUTE_VALUES_TLP 7677 N

XXXX XX_MTL_SYSTEM_ITEMS_21APR09 6732 N

XXXX XX_MTL_SYSTEM_ITEMS_INTERFACE 20406 Y

INV MMT_AAM_NOCOMPRESS 26366 Y

INV MMT_AAM_COMPRESS 4473 N

XXXX XX_ITEMS_IIM_CONV 3473 N

XXXX XX_ITEM_ATTRIBUTES_IIM_CONV 3552 N

XXXX XX_ITEMS_CONV 26796 Y

XXXX XX_ITEM_ATTRIBUTES_CONV 36963 Y

XXXX XX_ITEM_PINS_CONV 9522 N

XXXX TABLE_SITE_PART 20764 Y

XXXX TABLE_CONTRACT_ITM_LST 27265 Y

XXXX TABLE_SCM_VIEW 22274 Y

XXXX TABLE_C_SCHED_PD_AMT 55245 Y

XXXX XX_AS400_ISMTM_SERIAL 24565 Y

XXXX TABLE_CONDITION 12638 N

Now I’m going to do a full-scan on all of these and check V$SEGMENT_STATISTICS before and after. This isn’t the most elegant way to do things, but it serves the purpose.

SQL> create table my_segstat_1

2 as select * from v$segment_statistics

3 where (owner,object_name) in

4 (

5 select owner,table_name

6 from dba_tables

7 where blocks > 3200

8 and table_name not like ‘MLOG%’

9 and owner in

10 (select oracle_username from applsys.fnd_oracle_userid))

11 and statistic_name=’physical reads direct’

12 /

Table created.

Next, I’ll full-scan each of the 80 tables that have more than 3200 blocks:

SQL> spool scans.sql

SQL> select ‘select /*+ full (X) */ count(*) from ‘||owner||’.’||table_name||’ x ;’ q

2 from dba_tables

3 where blocks > 3200

4 and table_name not like ‘MLOG%’

5 and owner in

6 (select oracle_username from applsys.fnd_oracle_userid)

7 /

Query

——————————————————————————– ——————–

select /*+ full (X) */ count(*) from APPLSYS.AD_PATCH_COMMON_ACTIONS x;

select /*+ full (X) */ count(*) from AK.AK_REGION_ITEMS x;

select /*+ full (X) */ count(*) from ALR.ALR_ALERT_CHECKS x;

select /*+ full (X) */ count(*) from APPLSYS.AD_SNAPSHOT_FILES x;

select /*+ full (X) */ count(*) from APPLSYS.AD_TASK_TIMING x;

select /*+ full (X) */ count(*) from APPLSYS.FND_COLUMNS x;

select /*+ full (X) */ count(*) from APPLSYS.FND_CONCURRENT_REQUESTS x;

select /*+ full (X) */ count(*) from AR.HZ_HIERARCHY_NODES x;

select /*+ full (X) */ count(*) from AR.HZ_RELATIONSHIPS x;

select /*+ full (X) */ count(*) from APPLSYS.FND_LOBS x;

select /*+ full (X) */ count(*) from AX.AX_RULE_LINES x;

After flushing the buffer cache, I’ll run “scans.sql” to full-scan each of the 80 tables. I’m doing this in a production environment because I want to understand the boundary at which Oracle decides to flip on and off direct reads in a real-world scenario:

SQL> @scans

SQL> select /*+ full (X) */ count(*) from APPLSYS.AD_PATCH_COMMON_ACTIONS x;

COUNT(*)

———-

311051

SQL> select /*+ full (X) */ count(*) from AK.AK_REGION_ITEMS x;

COUNT(*)

———-

214997

SQL> select /*+ full (X) */ count(*) from ALR.ALR_ALERT_CHECKS x;

COUNT(*)

———-

459527

SQL> select /*+ full (X) */ count(*) from APPLSYS.AD_SNAPSHOT_FILES x;

COUNT(*)

———-

4590722

SQL> select /*+ full (X) */ count(*) from APPLSYS.AD_TASK_TIMING x;

COUNT(*)

———-

24780

<< many more examples deleted >>

When complete, I’ll create a second table based on V$SEGMENT_STATISTICS:

SQL> create table my_segstat_2

2 as select * from v$segment_statistics

3 where (owner,object_name) in

4 (

5 select owner,table_name

6 from dba_tables

7 where blocks > 3200

8 and table_name not like ‘MLOG%’

9 and owner in

10 (select oracle_username from applsys.fnd_oracle_userid))

11 and statistic_name=’physical reads direct’

12 /

Table created.

Finally, I’ll compare results.

SQL> select a.owner,a.object_name,a.value v1,b.value v2,

2 c.blocks ,

3 (case when b.value > a.value then ‘Direct’

4 else ‘Buffered’

5 end) dr

6 from my_segstat_1 a, my_segstat_2 b,

7 dba_tables c

8 where a.owner=b.owner

9 and a.object_name=b.object_name

10 and a.owner=c.owner

11 and a.object_name=c.table_name

12 order by c.blocks asc

13 /

Direct

Owner Object Direct Reads Start Direct Reads Start Blocks Buffered

————— —————————— —————— —————— ——— ———-

APPLSYS WF_ITEM_ACTIVITY_STATUSES 0 0 3237 Buffered

XXXX XX_AS400_IIMTM 0 0 3258 Buffered

APPLSYS AD_PATCH_RUN_BUGS 0 0 3379 Buffered

ALR ALR_ALERT_CHECKS 0 0 3394 Buffered

XXXX XX_ITEMS_IIM_CONV 0 0 3473 Buffered

INV MTL_INTERFACE_ERRORS 0 0 3520 Buffered

XXXX XX_ITEM_ATTRIBUTES_IIM_CONV 0 0 3552 Buffered

BEN BEN_PRTT_ENRT_RSLT_F 68925 72760 3835 Direct

APPLSYS WF_JAVA_DEFERRED 28392 28392 4056 Buffered

BOM BOM_COMPONENTS_B 20328 24400 4072 Direct

AR HZ_GEOGRAPHIES 21540 25848 4308 Direct

INV MMT_AAM_COMPRESS 43838 48233 4473 Direct

ICX ICX_SESSION_ATTRIBUTES 28196 32898 4705 Direct

AR HZ_GEOGRAPHY_IDENTIFIERS 23732 28481 4749 Direct

ICX ICX_CAT_ITEMS_CTX_HDRS_TLP 23745 28494 4749 Direct

APPLSYS FND_NEW_MESSAGES 57596 62832 5236 Direct

BOM BOM_INVENTORY_COMPS_INTERFACE 28545 34254 5709 Direct

APPLSYS FND_VIEW_COLUMNS 28458 34167 5709 Direct

APPLSYS FND_CONCURRENT_REQUESTS 67991 74172 6181 Direct

APPLSYS WF_DEFERRED 43596 43596 6228 Buffered

ICX DR$ICX_CAT_ITEMSCTXDESC_HDRS$I 31599 37922 6323 Direct

XXXX XX_MTL_SYSTEM_ITEMS_21APR09 100898 107630 6732 Direct

AK AK_REGION_ITEMS 76659 83628 6969 Direct

APPLSYS FND_VIEWS 35518 42628 7110 Direct

BEN BEN_PERSON_ACTIONS 36136 43420 7284 Direct

APPLSYS AD_FILES 37125 44550 7425 Direct

PO PO_ATTRIBUTE_VALUES_TLP 76698 84375 7677 Direct

ICX ICX_CAT_ATTRIBUTE_VALUES_TLP 77186 84911 7725 Direct

APPLSYS AD_TASK_TIMING 39604 47533 7929 Direct

APPLSYS JDR_COMPONENTS 39993 48001 8008 Direct

CZ CZ_IMP_PS_NODES 41646 50004 8358 Direct

AX AX_RULE_LINES 45300 54360 9060 Direct

XXXX XX_ITEM_PINS_CONV 47566 57088 9522 Direct

AR HZ_HIERARCHY_NODES 50955 61146 10191 Direct

APPLSYS FND_ENV_CONTEXT 61626 71897 10271 Direct

APPLSYS AD_PATCH_RUN_BUG_ACTIONS 52230 62676 10446 Direct

PO PO_ATTRIBUTE_VALUES 104588 115050 10462 Direct

APPLSYS FND_LOBS 52790 63348 10558 Direct

ICX ICX_CAT_ATTRIBUTE_VALUES 108107 118920 10813 Direct

APPLSYS FND_COLUMNS 56695 68034 11339 Direct

BEN BEN_ENRT_RT 598305 610313 12008 Direct

XXXX TABLE_CONDITION 62420 74904 12638 Direct

APPLSYS AD_FILE_VERSIONS 65615 78738 13123 Direct

APPLSYS AD_PATCH_COMMON_ACTIONS 68165 81798 13633 Direct

APPLSYS AD_SNAPSHOT_BUGFIXES 68465 82161 13696 Direct

INV MTL_ITEM_REVISIONS_TL 71058 85280 14222 Direct

APPLSYS AD_PROGRAM_RUN_TASK_JOBS 71350 85620 14270 Direct

BEN BEN_ELIG_PER_F 1683491 1699546 16055 Direct

BEN BEN_ELIG_PER_ELCTBL_CHC 742229 758778 16549 Direct

INV MTL_PENDING_ITEM_STATUS 106371 124115 17744 Direct

BEN BEN_REPORTING 133357 152408 19051 Direct

APPLSYS JDR_ATTRIBUTES 99912 119920 20008 Direct

XXXX XX_MTL_SYSTEM_ITEMS_INTERFACE 306090 326496 20406 Direct

XXXX TABLE_SITE_PART 102885 123462 20764 Direct

BOM CST_ITEM_COSTS 108160 129808 21648 Direct

XXXX TABLE_SCM_VIEW 110400 132480 22274 Direct

ICX ICX_CAT_ITEMS_CTX_DTLS_TLP 112433 134926 22493 Direct

EGO EGO_ITEM_TEXT_TL 117149 140598 23449 Direct

XXXX XX_AS400_ISMTM_SERIAL 121810 146172 24565 Direct

INV MMT_AAM_NOCOMPRESS 263660 290026 26366 Direct

INV MTL_SYSTEM_ITEMS_TL 365062 391428 26366 Direct

XXXX XX_ITEMS_CONV 133980 160776 26796 Direct

XXXX TABLE_CONTRACT_ITM_LST 135260 162312 27265 Direct

APPLSYS WF_ITEM_ATTRIBUTE_VALUES 167850 195825 27975 Direct

BEN BEN_ELIG_PER_OPT_F 2366120 2411138 29616 Direct

INV MTL_ITEM_REVISIONS_B 157594 189138 31544 Direct

XXXX XX_ITEM_ATTRIBUTES_CONV 184815 221778 36963 Direct

INV MTL_ITEM_BULKLOAD_RECS 193575 232290 38715 Direct

INV MTL_ITEM_REVISIONS_INTERFACE 196365 235638 39273 Direct

BOM BOM_STRUCTURES_B 286904 327165 40261 Direct

ENI ENI_OLTP_ITEM_STAR 216287 259576 43289 Direct

AR HZ_RELATIONSHIPS 613176 715372 51098 Direct

APPLSYS AD_SNAPSHOT_FILES 257080 359912 51416 Direct

XXXX TABLE_C_SCHED_PD_AMT 274595 329514 55245 Direct

INV MTL_ITEM_CATEGORIES 366272 439584 73312 Direct

APPLSYS DR$FND_LOBS_CTX$I 1070990 1285188 214198 Direct

INV MTL_SYSTEM_ITEMS_B 3279494 3498297 218803 Direct

INV MTL_SYSTEM_ITEMS_INTERFACE 3795720 4048768 253048 Direct

AZ AZ_DIFF_RESULTS 3607327 4328862 721535 Direct

AZ AZ_REPORTER_DATA 16843627 20194912 3351285 Direct

80 rows selected.

Based on our tests, the smallest table that did direct reads was at 3835 blocks, slightly over “_small_table_threshold” but not 5 times this value. These test results align with previous tests in this paper.

We did see buffered reads for a larger table, APPLSYS.WF_DEFERRED, at 6228 blocks. This is to not surprising considering the nature of the table – WF_DEFERRED would typically be cached because it’s always used.

Summary II

First, check out this table summarizing our direct read scenarios:

Here’s what I make of it:

– Oracle performs serial direct reads when the number of blocks exceeds the value of “_small_table_threshold”, which is 2% the size of the buffer cache

– The parallel query tests seemed to indicate that parallel direct reads occur if the number of blocks for the entire operation exceeds “_small_table_threshold”, not the maximum number of blocks retrieved for each PQ server/granule

– The well-documented, “5 times small table threshold” didn’t seem to hold up under my test cases, at least on 11.2.0.2 and 11.2.0.3. Oracle elected to do direct reads right at the small table threshold boundary. If anyone has any additional feedback I’ve love to hear it!

– The impact of cached, unmodified blocks had a very small impact on direct reads. Oracle adaptively turned off direct reads either very close to the total block count, or not at all.

– The impact of dirty blocks in the buffer cache and direct reads showed that Oracle elected to turn off direct reads when the number of blocks in the buffer cache reached about 45%. This is probably a very good thing, as it mitigates the penalty of object checkpoints (enq: KO – fast object checkpoint waits)

– Adaptive direct reads is an interesting topic – according to Oracle documentation and other sources, Oracle uses a number of different mechanisms to decide whether to do or not to do direct reads. The test cases in this white paper hopefully shed some light on a few of these test cases

– In this paper I haven’t talked at all about another set of hidden parameters, “_very_large_object_threshold” and “_very_large_partitioned_table”. These parameters control limits of the size of a table/partitioned-table to qualify for direct read.

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