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

From: Mladen Gogala <mladen@xxxxxxxxxxxxxxx>
To: oracle-l@xxxxxxxxxxxxx
Date: Mon, 1 Mar 2004 12:18:33 -0500
On 03/01/2004 12:09:48 PM, Guang Mei wrote:
> 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:
- 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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