In CBO, Statistic is critical important for Optimizer. Precise and timely statistics will truly reflect the data distribution and volume, and generate wiser SQL Execution Plan (SEP). After years of improvement, CBO is already widely accepted as the default and desirable solution for Oracle Optimizer.

As I mention in many blogs before, statistic is like the raw material for CBO optimizer. Without accurate statistic, it’s impossible to generate optimal SEP for CBO. So in CBO era, DBAs’ concern is about how to collect data statistic and finding out the proper frequency to collect them.

Since 10g, Oracle introduces a new feature of data statistic job to collect statistic automatically. The job will run at daily interval to collect statistic, and provide most database object statistic to CBO. The new feature solves many problems in most cases. The statistic collection job and mature dynamic sampling sweep the barrier of CBO’s usage.

1. Big Partition Table Statistic

Frankly speaking, Oracle internal functions are already enough for ordinary system requirement. But for some special cases, life would be tougher.

For many OLTP/OLAP systems, they usually load bulk of data into some extremely huge table (Mostly Partition Tables) in the night and some other business-free time, and then doing the processing work. The problem is that Oracle would have few chances to collect statistic after the data loading, which would influence the data volume and distribution information.

The problem would finally lead to the bad performance in the next processing work. In production environments, we often receive complains about some job processing is extremely slow. But after clearly examination, the old statistic is main reason. After collection, the job would be better. And then in the next loading, errors would happen again.

If we insert the statistic collection statement between data loading and processing, there would be some time consuming drawbacks for some big partition tables. For most partitions in these tables, the data is stable, and loading work only affects one or two partitions. But for normal partition statistic works, Oracle will collect all partition including inactive partitions, which consume a lot of resource.

In 11g, Oracle introduces a new feature named “Incremental Statistic”, which will only collect the partitions which are new or undergoing huge data changes. So it will save the time of collecting inactive partitions and make collection work shorter.

2. Environment Introduction

The new feature of “Incremental Statistic” is first introduced in the update edition of 10.2.0.4, and become formal in Oracle 11gR1. So we chose the Oracle 11gR2 as the test environment.

SQL> select * from v$version;

BANNER

--------------------------------------------------------------------------------

Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - Production

PL/SQL Release 11.2.0.1.0 - Production

CORE 11.2.0.1.0 Production

I prepare a normal list partition table named T_PART.

SQL> create table t_part

2 ( owner varchar2(100),

3 object_id number,

4 object_name varchar2(100),

5 create_date date

6 )

7 partition by list(owner)

8 (partition t_part_pub values ('PUBLIC'),

9 partition t_part_sys values ('SYS'),

10 partition t_part_big1 values ('APEX_030200','SYSMAN'),

11 partition t_part_big2 values ('ORDSYS','MDSYS'),

12 partition t_part_other values(default));

Table created

3. No-Incremental Statistic Behavior

Firstly, let’s see the default Oracle behavior. for partition statistic.

SQL> insert into t_part select owner, object_id, object_name, sysdate from dba_objects where owner in ('PUBLIC','ORDSYS');

30228 rows inserted

SQL> commit;

Commit complete

SQL> exec dbms_stats.gather_table_stats(user,'T_PART',cascade => true);

PL/SQL procedure successfully completed

SQL> select partition_name, NUM_ROWS, BLOCKS, LAST_ANALYZED, global_stats from dba_tab_partitions where table_owner='SYS' and table_name='T_PART';

PARTITION_NAME NUM_ROWS BLOCKS LAST_ANALYZED GLOBAL_STATS

--------------- ---------- ---------- -------------------- ------------

T_PART_PUB 27696 197 2012/10/9 22:08:11 YES

T_PART_SYS 0 0 2012/10/9 22:08:11 YES

T_PART_BIG1 0 0 2012/10/9 22:08:10 YES

T_PART_BIG2 2532 20 2012/10/9 22:08:10 YES

T_PART_OTHER 0 0 2012/10/9 22:08:10 YES

The default behavior. is that Oracle will collect all partitions according to the last_analyzed column in view dba_tab_partition.

Doing some modification focus on some partitions, let’s see the collection results.

SQL> insert into t_part select owner, object_id, object_name, sysdate from dba_objects where owner in ('SYS','APEX_030200');

33241 rows inserted

SQL> commit;

Commit complete

SQL> exec dbms_stats.gather_table_stats(user,'T_PART',cascade => true);

PL/SQL procedure successfully completed

SQL> select partition_name, NUM_ROWS, BLOCKS, LAST_ANALYZED, global_stats from dba_tab_partitions where table_owner='SYS' and table_name='T_PART';

PARTITION_NAME NUM_ROWS BLOCKS LAST_ANALYZED GLOBAL_STATS

--------------- ---------- ---------- -------------------- ------------

T_PART_PUB 27696 197 2012/10/9 22:13:31 YES

T_PART_SYS 30835 197 2012/10/9 22:13:31 YES

T_PART_BIG1 2406 20 2012/10/9 22:13:30 YES

T_PART_BIG2 2532 20 2012/10/9 22:13:31 YES

T_PART_OTHER 0 0 2012/10/9 22:13:31 YES

The insert statement only affects partition T_PART_SYS and T_PART_BIG1. But when we run the collection statement, all partitions will getting involved in the collection work.

4. Incremental Statistic Setting

It’s easy to change the statistic strategy for big partition table in Oracle 11g. Using the package dbms_stats will help setting the parameters.

The default strategy for Oracle partition table is not incremental setting. Only three parameters will affect the behavior.

SQL> select dbms_stats.get_prefs('PUBLISH','SYS','T_PART') from dual;

DBMS_STATS.GET_PREFS('PUBLISH'

--------------------------------------------------------------------------------

TRUE

SQL> select dbms_stats.get_prefs('INCREMENTAL','SYS','T_PART') from dual;

DBMS_STATS.GET_PREFS('INCREMEN

--------------------------------------------------------------------------------

FALSE

SQL> select dbms_stats.get_prefs('GRANULARITY','SYS','T_PART') from dual;

DBMS_STATS.GET_PREFS('GRANULAR

--------------------------------------------------------------------------------

AUTO

Parameter “PUBLISH” is used to determinate whether CBO use the newest statistic when new statistic coming. It relates to pending statistic feature in most cases. And we need to ensure its value is true (Default Value).

Parameter “Incremental” default value is false, which means it would not adopt incremental statistic strategy by default.

Parameter “GRANULARITY” is not related to incremental statistic strategy in Oracle 11g, but related in Oracle 10gR2. So we need to keep it “AUTO” in Oracle 11g environment.

The estimated_precent should be keep to “AUTO”, in order to maintain the incremental statistic.

In the next section, we will see the effect of the new feature.

5. Incremental Statistic Behavior

First, we need to change the setting for Big partition table.

SQL> exec dbms_stats.set_table_prefs(user,'T_PART',pname => 'INCREMENTAL',pvalue => 'TRUE');

PL/SQL procedure successfully completed

SQL> select dbms_stats.get_prefs('INCREMENTAL','SYS','T_PART') from dual;

DBMS_STATS.GET_PREFS('INCREMEN

--------------------------------------------------------------------------------

TRUE

Do some modification work to T_PART, and see the effect of collection.

SQL> select partition_name, NUM_ROWS, BLOCKS, LAST_ANALYZED, global_stats from dba_tab_partitions where table_owner='SYS' and table_name='T_PART';

PARTITION_NAME NUM_ROWS BLOCKS LAST_ANALYZED GLOBAL_STATS

--------------- ---------- ---------- -------------------- ------------

T_PART_PUB 27696 197 2012/10/9 22:13:31 YES

T_PART_SYS 30835 197 2012/10/9 22:13:31 YES

T_PART_BIG1 2406 20 2012/10/9 22:13:30 YES

T_PART_BIG2 2532 20 2012/10/9 22:13:31 YES

T_PART_OTHER 0 0 2012/10/9 22:13:31 YES

SQL> insert into t_part select owner, object_id, object_name, sysdate from dba_objects where owner not in ('SYS','APEX_030200','PUBLIC','SYSMAN','ORDSYS','MDSYS');

3465 rows inserted

SQL> insert into t_part select owner, object_id, object_name, sysdate from dba_objects where owner in ('SYS');

30835 rows inserted

SQL> commit;

Commit complete

SQL> exec dbms_stats.gather_table_stats(user,'T_PART',cascade => true);

PL/SQL procedure successfully completed

SQL> select partition_name, NUM_ROWS, BLOCKS, LAST_ANALYZED, global_stats from dba_tab_partitions where table_owner='SYS' and table_name='T_PART';

PARTITION_NAME NUM_ROWS BLOCKS LAST_ANALYZED GLOBAL_STATS

--------------- ---------- ---------- -------------------- ------------

T_PART_PUB 27696 197 2012/10/9 22:29:26 YES

T_PART_SYS 61670 398 2012/10/9 22:29:21 YES

T_PART_BIG1 2406 20 2012/10/9 22:29:29 YES

T_PART_BIG2 2532 20 2012/10/9 22:29:22 YES

T_PART_OTHER 3465 23 2012/10/9 22:29:28 YES

The result is weird. Only two partitions are affected in the insert serial, but after collection, we find all partitions are collected. Not the things should be~

Let’s do more continue.

--Delete only One Partition Data

SQL> delete t_part where wner='SYS';

61670 rows deleted

SQL> commit;

Commit complete

SQL> exec dbms_stats.gather_table_stats(user,'T_PART');

PL/SQL procedure successfully completed

SQL> select partition_name, NUM_ROWS, BLOCKS, LAST_ANALYZED, global_stats from dba_tab_partitions where table_owner='SYS' and table_name='T_PART';

PARTITION_NAME NUM_ROWS BLOCKS LAST_ANALYZED GLOBAL_STATS

--------------- ---------- ---------- -------------------- ------------

T_PART_PUB 27696 197 2012/10/9 22:29:26 YES

T_PART_SYS 0 398 2012/10/9 22:42:47 YES

T_PART_BIG1 2406 20 2012/10/9 22:29:29 YES

T_PART_BIG2 2532 20 2012/10/9 22:29:22 YES

T_PART_OTHER 3465 23 2012/10/9 22:29:28 YES

The desired result comes out. Only the affected partitions is collected according to the last_analyzed column, and the table analyzed time is also changed.

SQL> select table_name, last_analyzed from dba_tables where wner='SYS' and table_name='T_PART';

TABLE_NAME LAST_ANALYZED

------------------------------ --------------------

T_PART 2012/10/9 22:42:49

If we affect more than one partition at once, how things would be.

SQL> insert into t_part select owner, object_id, object_name, sysdate from dba_objects where owner in ('PUBLIC');

27696 rows inserted

SQL> insert into t_part select owner, object_id, object_name, sysdate from dba_objects where owner in ('APEX_030200');

2406 rows inserted

SQL> commit;

Commit complete

SQL> exec dbms_stats.gather_table_stats(user,'T_PART',cascade => true);

PL/SQL procedure successfully completed

SQL> select partition_name, NUM_ROWS, BLOCKS, LAST_ANALYZED, global_stats from dba_tab_partitions where table_owner='SYS' and table_name='T_PART';

PARTITION_NAME NUM_ROWS BLOCKS LAST_ANALYZED GLOBAL_STATS

--------------- ---------- ---------- -------------------- ------------

T_PART_PUB 55392 388 2012/10/9 22:47:07 YES

T_PART_SYS 30874 398 2012/10/9 22:45:21 YES

T_PART_BIG1 4812 36 2012/10/9 22:47:08 YES

T_PART_BIG2 2532 20 2012/10/9 22:29:22 YES

T_PART_OTHER 3465 23 2012/10/9 22:29:28 YES

The truncate statement will affect the partitions and let’s see the results.

SQL> truncate table t_part;

Table truncated

SQL> exec dbms_stats.gather_table_stats(user,'T_PART',cascade => true);

PL/SQL procedure successfully completed

SQL> select partition_name, NUM_ROWS, BLOCKS, LAST_ANALYZED, global_stats from dba_tab_partitions where table_owner='SYS' and table_name='T_PART';

PARTITION_NAME NUM_ROWS BLOCKS LAST_ANALYZED GLOBAL_STATS

--------------- ---------- ---------- -------------------- ------------

T_PART_PUB 0 0 2012/10/9 22:55:31 YES

T_PART_SYS 0 0 2012/10/9 22:55:31 YES

T_PART_BIG1 0 0 2012/10/9 22:55:31 YES

T_PART_BIG2 0 0 2012/10/9 22:55:31 YES

T_PART_OTHER 0 0 2012/10/9 22:55:31 YES

SQL> insert into t_part select owner, object_id, object_name, sysdate from dba_objects where owner in ('SYS','APEX_030200');

33280 rows inserted

SQL> commit;

Commit complete

SQL> exec dbms_stats.gather_table_stats(user,'T_PART',cascade => true);

PL/SQL procedure successfully completed

SQL> select partition_name, NUM_ROWS, BLOCKS, LAST_ANALYZED, global_stats from dba_tab_partitions where table_owner='SYS' and table_name='T_PART';

PARTITION_NAME NUM_ROWS BLOCKS LAST_ANALYZED GLOBAL_STATS

--------------- ---------- ---------- -------------------- ------------

T_PART_PUB 0 0 2012/10/9 22:55:31 YES

T_PART_SYS 30874 202 2012/10/9 22:58:05 YES

T_PART_BIG1 2406 20 2012/10/9 22:58:07 YES

T_PART_BIG2 0 0 2012/10/9 22:55:31 YES

T_PART_OTHER 0 0 2012/10/9 22:55:31 YES

Ok, the result is correct.

Only one thing we still wandering: Why we first switch the table to incremental statistic mode, and then we do some modification on some partitions. After collection, we found Oracle will collect the statistic for all partition, and then incremental will happen in the next collection.

The reasonable explanation is: Like Oracle incremental backup strategy, although you have a full mode backup, Oracle would do the incremental full backup work at first time. As for incremental statistic, things are similar. Oracle first needs a full incremental statistic, and then collects the increments.

6. Conclusion

The advantage of incremental statistic is that they can collect only active partitions’ data in the shorter time. The inactive partitions which do not have significant data modification operation will not getting involved in the collection work.

For the load data and then process jobs, it is the desirable collection strategy.

Applying “Incremental Statistic” for Oracle Big Partition Table

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