EXP PERFORMANCE PROBLEM IN QUERY OVER EXU8COLU (Doc ID 353779.1)

Last updated on NOVEMBER 12, 2010

Applies to:

Enterprise Manager for RDBMS
Information in this document applies to any platform.

Symptoms

Exp has performance problems and the long running query is the following:

SELECT TOWNER, TNAME, NAME, LENGTH, PRECISION, SCALE, TYPE, ISNULL,
CONNAME, COLID, INTCOLID, SEGCOLID, COMMENT$, DEFAULT$, DFLTLEN,
ENABLED, DEFER, FLAGS, COLPROP, ADTNAME, ADTOWNER, CHARSETID,
CHARSETFORM, FSPRECISION, LFPRECISION, CHARLEN, TFLAGS, 100
FROM SYS.EXU8COLU WHERE TOBJID = :1 ORDER BY INTCOLID;

Looking at the execution plan of this query we can see that it uses a 'MERGE JOIN CARTESIAN'.

If we explain this query in 9i and 10g databases we can check the following:

1) In 9i there is no 'MERGE JOIN CARTESIAN'

2) In 10g, if we use Cost based optimizer, we have OPTIMIZER_MODE=CHOOSE or
    ALL_ROWS, then there is the MERGE JOIN CARTESIAN' in the access plan

3) In 10g, if we set OPTIMIZER_MODE=RULE, the the right access path is used and there is no
    'MERGE JOIN CARTESIAN' . When we use the RULE based optimizer the access plan is
    similar to the 9i one.

This problem only reproduces if there are statistics gathered for the dictionary schema.

Changes

No prerequisite

Cause

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