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Question and Answer

Connor McDonald

Thanks for the question, donki.

Asked: February 07, 2018 - 9:40 pm UTC

Last updated: December 12, 2018 - 3:40 am UTC

Version: Oracle 11g

Viewed 10K+ times! This question is

You Asked

Hi,
I'm trying to understand the trigger that changes the column value STALE_STATS in DBA_TAB_STATISTICS from NO to YES.
I have duplicated a table, gathered stats, deleted 35% of the records, and the stats aren't STALE='YES'.
I'd appreciate your insight on that one;

--CHECKING COUNT OF ORIGINAL TABLE
SELECT COUNT(1) FROM ALL_OBJECTS --1051227

--CREATING A DUP
CREATE TABLE TMP.TMP_ALL_OBJECTS AS SELECT * FROM all_objects

--CHECKING THE DUP IS IDENTICAL
SELECT COUNT(1) FROM TMP.TMP_ALL_OBJECTS --1051227

--CHECKING FOR STALE_STATS IN NEW TABLE
SELECT STALE_STATS FROM DBA_TAB_STATISTICS WHERE TABLE_NAME = 'TMP_ALL_OBJECTS' --NULL

--GATHERING STATS
BEGIN
 dbms_stats.gather_table_stats('TMP','TMP_ALL_OBJECTS',cascade=>TRUE);
END;

--CHECKING FOR STALE_STATS AFTER STATS GATHERING
SELECT STALE_STATS FROM ALL_TAB_STATISTICS WHERE TABLE_NAME = 'TMP_ALL_OBJECTS' --NO

--DELETEING 35% OF RECORDS
DELETE FROM CIT_INFRA_REPOS.TMP_ALL_OBJECTS WHERE OWNER='FIXED_INCOME';
COMMIT;

SELECT COUNT(1) FROM CIT_INFRA_REPOS.TMP_ALL_OBJECTS --651677

--checking the threshold
SELECT DBMS_STATS.GET_PREFS('STALE_PERCENT', 'TMP', 'TMP_ALL_OBJECTS') STALE_PERCENT FROM DUAL; --10

--CHECKING STALE_STATS 
SELECT STALE_STATS FROM ALL_TAB_STATISTICS WHERE TABLE_NAME = 'TMP_ALL_OBJECTS' --NO


Thanks in advance

and Connor said...

You were close :-)

We don't *immediately* up date the staleness, because if we did, we would have to do it every time someone ran a DML statement. So we track it in memory, and flush it to the dictionary from time to time.

Or....you can flush it manually

SQL> SELECT COUNT(1) FROM ALL_OBJECTS;

  COUNT(1)
----------
     73853

1 row selected.

SQL> CREATE TABLE TMP_ALL_OBJECTS AS SELECT * FROM all_objects;

Table created.

SQL> SELECT COUNT(1) FROM TMP_ALL_OBJECTS;

  COUNT(1)
----------
     73853

1 row selected.

SQL> SELECT STALE_STATS FROM DBA_TAB_STATISTICS WHERE TABLE_NAME = 'TMP_ALL_OBJECTS';

STA
---
NO

1 row selected.

SQL> exec dbms_stats.gather_table_stats('','TMP_ALL_OBJECTS',cascade=>TRUE);

PL/SQL procedure successfully completed.

SQL> SELECT STALE_STATS FROM ALL_TAB_STATISTICS WHERE TABLE_NAME = 'TMP_ALL_OBJECTS';

STA
---
NO

1 row selected.

SQL> DELETE FROM TMP_ALL_OBJECTS WHERE rownum <= 30000;

30000 rows deleted.

SQL> COMMIT;

Commit complete.

SQL> SELECT COUNT(1) FROM TMP_ALL_OBJECTS;

  COUNT(1)
----------
     43853

1 row selected.

SQL> SELECT DBMS_STATS.GET_PREFS('STALE_PERCENT', user, 'TMP_ALL_OBJECTS') STALE_PERCENT FROM DUAL;

STALE_PERCENT
----------------------------------------------------------------------------------------------------------------------------------
10

1 row selected.

SQL> SELECT STALE_STATS FROM ALL_TAB_STATISTICS WHERE TABLE_NAME = 'TMP_ALL_OBJECTS';

STA
---
NO

1 row selected.

<b>SQL> exec dbms_stats.flush_database_monitoring_info;</b>

PL/SQL procedure successfully completed.

SQL> SELECT STALE_STATS FROM ALL_TAB_STATISTICS WHERE TABLE_NAME = 'TMP_ALL_OBJECTS';

STA
---
YES

1 row selected.

SQL>
SQL>


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Comments

Great answer

Yaniv, February 08, 2018 - 3:04 am UTC

Thanks.
But what is the definition of
"So we track it in memory, and flush it to the dictionary from time to time. "

- Is "time to time" is a configurable attribute ?
- Can we know what is the time period ?


Connor McDonald
February 09, 2018 - 2:17 am UTC

From memory, I think its 15mins

Additional point

A reader, February 09, 2018 - 7:46 am UTC

Hi,

https://docs.oracle.com/cd/E11882_01/appdev.112/e40758/d_stats.htm#ARPLS68568

Because the GATHER_*_STATS procedures internally flush monitoring information, it is not necessary to run this procedure before gathering the statistics.

This means you can just use GATHER STALE/AUTO, without manually flushing first.

regards,
Connor McDonald
February 09, 2018 - 1:27 pm UTC

Thanks for that - I was unaware of that, but of course it makes perfect sense.

Question

Shira, December 06, 2018 - 8:25 am UTC

Do the optimizer lay on statistics staleness and how?

Connor McDonald
December 12, 2018 - 3:40 am UTC

lay ?
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