Oracle-L: Re: col_usage$ question

From: Tanel Poder <tanel.poder.003_at_mail.ee>
Date: Tue, 18 Nov 2003 10:04:27 -0800
Message-ID: <F001.005D7111.20031118100427@fatcity.com>

col_usage$ questionHi!

If you describe this table then you see that this table stores column usage information in filter and join predicates for database objects. >From describe, you see there are several filter and join conditions tracked for an object's (obj#) columns (intcol#). You can join them to col$ table for example.

During shutdown, the session executing shutdown, writes the column usage statistics to col_usage$. During normal operations, it's SMON who's doing that over regular intervals.

You can disable collecting these statistics by setting _column_tracking_level to 0.

I don't really see where CBO could use those statistics for speeding up statement execution, because during execution CBO knows all the predicates & statement structure anyway. But it is probably useful for various 10g's advisories, which can make you recommendations based on how the tables (columns) are used. Also, it might help automatic statistics gathering to determine which stats need to be updated or not (this gather stale stuff).

Tanel.

Original Message ----- From: Jamadagni, Rajendra To: Multiple recipients of list ORACLE-L Sent: Tuesday, November 18, 2003 6:29 PM Subject: col_usage$ question

Does anyone know what this table (sys.col_usage$) is used for? To me it sounds like something that CBO might appreciate ... but any ideas? It is referenced by dbms_stats and dbms_stats_internal packages ...

Thanks in advance
Raj

Rajendra dot Jamadagni at nospamespn dot com All Views expressed in this email are strictly personal. QOTD: Any clod can have facts, having an opinion is an art !

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Author: Tanel Poder
INET: tanel.poder.003_at_mail.ee

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