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

From: "zhu chao" <chao_ping@xxxxxxxxxxx>
To: <oracle-l@xxxxxxxxxxxxx>
Date: Tue, 2 Mar 2004 10:38:54 +0800
hi, guang:
    If all other SQL performs well in the new 9i instance, I would consider
using dbms_stats to transfer the statistics from 8i to 9i and check if the
explain plan goes better, if it still does not work, I would use HINT. Since
using  use_hash get the good plan, what do you means by saying:  sort
opration costs too high? Is it the actual SQL cost more time or just the
explain cost get high? IF only the explain cost high, I won't care about it
at all.
    Do you use workarea_size_policy=auto in 9i?
    If more SQLs get bad, I would consider using
optimizer_features_enable=8.1.7 and other optimizer related parameter the
same as 8i.

Regards
Zhu Chao

----- Original Message -----
From: "Guang Mei" <gmei@xxxxxxxxxx>
To: "Oracle-L-freelists" <oracle-l@xxxxxxxxxxxxx>
Sent: Tuesday, March 02, 2004 1:09 AM
Subject: query slow in 9i, but not slow in 8iut


> 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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Follow-Ups:
- Re: query slow in 9i, but not slow in 8i
  - From: Mladen Gogala
- Re: query slow in 9i, but not slow in 8i
  - From: Guang Mei
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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