Re: Synonyms can be VERY bad for performance

From: A. Bardeen <abardeen1_at_yahoo.com>
Date: Wed, 31 Oct 2001 15:10:11 -0800
Message-ID: <F001.003B9E6A.20011031153018@fatcity.com>

Greg,

Hey! I resemble that remark ;)

The data dictionary views are optimized to use the RBO or are heavily hinted to force a specific access path.  ALL_ROWS and FIRST_ROWS force the CBO to be used so a different access path may be taken resulting in poor performance.

Obligatory notes:

Note: 35272.1 "Is ANALYZE on the Data Dictionary Supported (TABLES OWNED BY SYS)?"

Note: 35934.1 "TECH: Cost Based Optimizer - Common Misconceptions and Issues"

Note: 66484.1 "Which Optimizer is used"

HTH,

• Anita
  • Greg Moore <sqlgreg_at_pacbell.net> wrote:
    > > Because we were in first_rows, queries
    > > against the data dictionary were
    > > optimized in first_rows mode rather than rule.
    > > This was despite us not having any statistics
    > > on system or sys objects.
    >
    > Is this standard behavior?
    >
    > What about the warnings not to analyze SYS because
    > it's optimized for Rule?
    > Is it true that simply invoking first_rows means
    > that's out the window and
    > you get the CBO used on SYS anyway, even if there
    > are no statistics?
    >
    > I get the feeling we're going to see an email from
    > Anita saying this is bug
    > number XXX, fixed in patch YYY, and we can read all
    > about it in Note ZZZ.
    >
    > --
    > Please see the official ORACLE-L FAQ:
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    > --
    > Author: Greg Moore
    > INET: sqlgreg_at_pacbell.net
    >
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-- 
Author: A. Bardeen
  INET: abardeen1_at_yahoo.com

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Received on Wed Oct 31 2001 - 17:10:11 CST