Keeping Optimizer statistics up to date

This script can be scheduled to run regularly to monitor table usage and keep statistics on schema objects current.

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In order for Oracle to use the Cost Based Optimizer (CBO) effectively, schema objects must have current statistics. Instead of analyzing all user objects regularly, which is costly, in 8i and above you can have Oracle monitor table usage. Utilizing the dbms_stats package you then only need to analyze the table when there has been significant change. The following script can be scheduled to run regularly. Note: You must gather statistics...

initially and issued the "alter

monitoring;" statement for this to work. Also, when issuing the statement in SQL*Plus remember to set serveroutput on.

I based the framework (cycling through all non-sys and system schemas) on an article on asktom.oracle.com. I also used information on Metalink (Oracle documentation) on how the dbms_stats stored procedure works.

a dbms_stats.objecttab;
  b dbms_stats.objecttab;
BEGIN
 FOR s in (select distinct(owner) schema
             from dba_tables
             where owner not in ('SYS','SYSTEM'))
   LOOP
        dbms_output.put_line('Analyzing the entire '||s.schema||' schema');
--List tables with stale statistics
        dbms_stats.gather_schema_stats(ownname => s.schema,
                                        options => 'LIST STALE',
                                        objlist => b);
        for i in 1 .. b.count
          loop
             dbms_output.put_line( 'Stale Stats: '||b(i).objName );
         end loop;
-- Analyze objects with stale statistics
        dbms_stats.gather_schema_stats(ownname => s.schema,
                                        method_opt => 'FOR ALL INDEXED COLUMNS',
                                        degree => 4,
                                        cascade => TRUE,
                                        options => 'GATHER STALE');
--List any objects with missing statistics
        dbms_stats.gather_schema_stats(ownname => s.schema,
                                        options => 'LIST EMPTY',
                                        objlist => a);
        for i in 1 .. a.count
          loop
             dbms_output.put_line( 'Stats missing: '||a(i).objType );
             dbms_output.put_line( 'Object Name: '||a(i).objName );
          end loop;
   End Loop;
END;

Reader Feedback

Brett F. writes: It should be fairly obvious that this tip won't work plugged directly into SQL*PLUS. I tried it anyway just in case I was being dense and got a slew of errors. It is not clear from the context how this should be run, although the author seems to have intended it to be run in this way as he mentions that you should "set serveroutput on". Here are the errors I received when I ran it "as is":

ERROR at line 10:
ORA-06550: line 10, column 52:
PLS-00201: identifier 'B' must be declared
ORA-06550: line 8, column 9:
PL/SQL: Statement ignored
ORA-06550: line 11, column 23:
PLS-00201: identifier 'B.COUNT' must be declared
ORA-06550: line 11, column 9:
PL/SQL: Statement ignored
ORA-06550: line 25, column 52:
PLS-00201: identifier 'A' must be declared
ORA-06550: line 23, column 9:
PL/SQL: Statement ignored
ORA-06550: line 26, column 23:
PLS-00201: identifier 'A.COUNT' must be declared
ORA-06550: line 26, column 9:
PL/SQL: Statement ignored

I'm running Oracle 8.1.7.0.0.

Jiayong S. writes regarding the question above: It will work perfectly if you just add DECLARE at the beginning when executing it in SQL*PLUS.

The author. responds: I should have added that the script is the text for a stored procedure. Therefore, the following line should be added to the beginning of the script:

create or replace procedure GET_STATS  as

To execute in SQL*Plus after you create the procedure:

set serveroutput on
exec dbms_output.enable(100000);  --may be added to the beginning of the script 
                                  --to ensure a large enough output buffer
exec get_stats;

This will then provide you with like this:

Analyzing the entire USER1 schema
Stale Stats: TABLE1
Stale Stats: TABLE2
Stale Stats: TABLE3
Stats missing: INDEX
Object Name: TABLE4_PK
Stats missing: INDEX
Object Name: TABLE5_PK
Analyzing the entire USER2 schema
Analyzing the entire USER3 schema
Stale Stats: TABLE1
Analyzing the entire USER4 schema

I am using OEM Jobs to run this every day on my system. I developed this in 8.1.7 so it should work for Brett. He just needs to be aware that in 8.1.7, it takes about 4 hours for SMON to mark table stats as stale. This has been improved to 15 minutes in 9i. One other thing to note is that if you get the following error:

ERROR at line 1:
ORA-20000: ORU-10027: buffer overflow, limit of 2000 bytes
ORA-06512: at "SYS.DBMS_OUTPUT", line 91
ORA-06512: at "SYS.DBMS_OUTPUT", line 58
ORA-06512: at "DGIBBS.GET_STATS", line 15
ORA-06512: at line 1

You need to enable a larger buffer (though there is a max size, I just can't remember off the top of my head -- it's in the manual).

Sorry I wasn't clear enough.

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This was first published in November 2002

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