Re: query slow in 9i, but not slow in 8i

From: "Juan Cachito Reyes Pacheco" <jreyes@xxxxxxxxxxxxxxxx>
To: <oracle-l@xxxxxxxxxxxxx>
Date: Mon, 1 Mar 2004 13:36:54 -0400
Maybe you can try doing this, to be sure every thing has statistics
EXEC DBMS_STATS.GATHER_DATABASE_STATS();

----- Original Message ----- 
From: "Guang Mei" <gmei@xxxxxxxxxx>
To: <oracle-l@xxxxxxxxxxxxx>
Sent: Monday, March 01, 2004 1:22 PM
Subject: RE: query slow in 9i, but not slow in 8i


> Yes, The statistics is update to date on all the tables (in both 8i and
9i).
> I ran
>
> execute DBMS_STATS.GATHER_TABLE_STATS(<owenr>, <table_name> ,cascade =>
> TRUE);
>
> on all the tables in the schema.
>
>
> Guang
>
> -----Original Message-----
> From: oracle-l-bounce@xxxxxxxxxxxxx
> [mailto:oracle-l-bounce@xxxxxxxxxxxxx]On Behalf Of Juan Cachito Reyes
> Pacheco
> Sent: Monday, March 01, 2004 12:11 PM
> To: oracle-l@xxxxxxxxxxxxx
> Subject: Re: query slow in 9i, but not slow in 8i
>
>
> If you have statictis up to date with dbms package.
>
> We had a problem with the new views cbo feature
> try to use the /*+ NO_MERGE */ hint,
> if this is the same problem we had , this will fix that.
> Otherwise no idea.
>
> ----- Original Message -----
> From: "Guang Mei" <gmei@xxxxxxxxxx>
> To: "Oracle-L-freelists" <oracle-l@xxxxxxxxxxxxx>
> Sent: Monday, March 01, 2004 1:09 PM
> Subject: query slow in 9i, but not slow in 8i
>
>
> > Hi:
> >
> > I have a query which gave two very different explain plan on 8173 and
> 9204.
> > The two instances (on two separate Sun Solaris boxes) both have the same
> > db_file_multiblock_read_count (8), block_size (8k), sort_area_size
> > (90000000) and sort_area_retained_size (9000000). I narrowed down the
part
> > which causeed this:
> >
> > select distinct accession2, id from (
> > select  accession2,
> >         Identifier.id
> > from    mt.External_accession, mt.identifier
> > where   external_accession.SEQTABLEID = identifier.seqtabid and
> >         identifier.type != 'A' and
> >         identifier.speciesid in
> > (24,31,2,19,18,17,23,21,27,32,20,34,30,22,25,26,28,29)
> > );
> >
> >
> > -- on 9204 (slow):
> >
> > Execution Plan
> > ----------------------------------------------------------
> >    0      SELECT STATEMENT Optimizer=CHOOSE (Cost=118228 Card=8046044
> >           Bytes=225289232)
> >
> >    1    0   SORT (UNIQUE) (Cost=118228 Card=8046044 Bytes=225289232)
> >    2    1     MERGE JOIN (Cost=1102 Card=8046044 Bytes=225289232)
> >    3    2       TABLE ACCESS (BY INDEX ROWID) OF 'EXTERNAL_ACCESSION'
> >           (Cost=826 Card=28898835 Bytes=404583690)
> >
> >    4    3         INDEX (FULL SCAN) OF 'EXTACC_SEQTABID_INDEX' (NON-UN
> >           IQUE) (Cost=26 Card=28898835)
> >
> >    5    2       SORT (JOIN) (Cost=276 Card=51045 Bytes=714630)
> >    6    5         TABLE ACCESS (FULL) OF 'IDENTIFIER' (Cost=96 Card=51
> >           045 Bytes=714630)
> >
> >
> > -- on 8173 (not slow):
> >
> > Execution Plan
> > ----------------------------------------------------------
> >    0      SELECT STATEMENT Optimizer=CHOOSE (Cost=77324 Card=6490972 B
> >           ytes=188238188)
> >
> >    1    0   SORT (UNIQUE) (Cost=77324 Card=6490972 Bytes=188238188)
> >    2    1     HASH JOIN (Cost=41347 Card=6490972 Bytes=188238188)
> >    3    2       TABLE ACCESS (FULL) OF 'IDENTIFIER' (Cost=95 Card=3892
> >           9 Bytes=545006)
> >
> >    4    2       TABLE ACCESS (FULL) OF 'EXTERNAL_ACCESSION' (Cost=4111
> >           4 Card=38102138 Bytes=571532070)
> >
> >
> > It looks like 9i thinks MERGE JOIN is better than HASH JOIN for the
> subquery
> > (which is fine). The problem is that when I have "select distinct
> > accession2, id from ..." from the outside, the "SORT (UNIQUE)" part
makes
> > the cost 100 times higher in 9204 (from 1102 to 118228) while in 8173 it
> > only increases the cost less than two times (from 41347 to 77324).
> >
> > I tried the a couple of ways in 9i, such as adding a hint /*+
> > USE_HASH(identifier) */ in the subquery. This did results in the
subquery
> > using hash join instead of merge join, but it did not solve the problem.
> The
> > cost still went to 100 times higher when I put "select distinct
> accession2,
> > id from ..." there.  I aslo changed the init parameter
> > "optimizer_max_permutations" to 80000 for the session but it did not
help
> > either.
> >
> > So my question is:
> >
> > 1. What is the reason that in 9204 the sort opration costs that high
while
> > it does not in 8173?
> >
> > 2. Any work around?
> >
> > TIA.
> >
> > Guang
> >
> >
> >
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References:
- RE: query slow in 9i, but not slow in 8i
  - From: Guang Mei
Re: query slow in 9i, but not slow in 8i

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