Обсуждение: Performance of full outer join in 8.3
Hi list,
we have just migrated one of our databases from 8.2.12 to 8.3.7. We now
experience a strange problem: A query that was really fast on the 8.2
server is now much slower on the 8.3 server (1 ms vs. 60 sec). I had a
look at the query plan and it is completely different. Both servers run
on the same machine. The configuration (planner constants etc.) is
identical. The database has been vacuum analyzed after the migration. So
why the difference?
This is the query:
select isin from ts_frontend.attachment_isins full OUTER JOIN
ts_frontend.rec_isins using (attachment,isin) WHERE attachment=2698120
GROUP BY isin limit 1000;
Here is the explain analyze in 8.2:
QUERY
PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=826.44..826.61 rows=17 width=32) (actual time=0.163..0.172
rows=2 loops=1)
-> HashAggregate (cost=826.44..826.61 rows=17 width=32) (actual
time=0.159..0.162 rows=2 loops=1)
-> Merge Full Join (cost=799.62..826.40 rows=17 width=32)
(actual time=0.122..0.144 rows=2 loops=1)
Merge Cond: (("outer"."?column3?" = "inner"."?column3?")
AND (attachment_isins.attachment = rec_isins.attachment))
Filter: (COALESCE(attachment_isins.attachment,
rec_isins.attachment) = 2698120)
-> Sort (cost=13.39..13.74 rows=138 width=20) (actual
time=0.065..0.067 rows=1 loops=1)
Sort Key: (attachment_isins.isin)::bpchar,
attachment_isins.attachment
-> Index Scan using
attachment_isins_attachment_idx on attachment_isins (cost=0.00..8.49
rows=138 width=20) (actual time=0.042..0.047 rows=1 loops=1)
Index Cond: (attachment = 2698120)
-> Sort (cost=786.23..794.80 rows=3429 width=20)
(actual time=0.045..0.049 rows=2 loops=1)
Sort Key: (rec_isins.isin)::bpchar,
rec_isins.attachment
-> Index Scan using idx_rec_isins_attachment on
rec_isins (cost=0.00..584.89 rows=3429 width=20) (actual
time=0.019..0.024 rows=2 loops=1)
Index Cond: (attachment = 2698120)
Total runtime: 0.302 ms
(14 rows)
And this is the 8.3 plan:
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=345890.35..345900.35 rows=1000 width=26) (actual
time=53926.706..53927.071 rows=2 loops=1)
-> HashAggregate (cost=345890.35..346296.11 rows=40576 width=26)
(actual time=53926.702..53927.061 rows=2 loops=1)
-> Merge Full Join (cost=71575.91..345788.91 rows=40576
width=26) (actual time=10694.727..53926.559 rows=2 loops=1)
Merge Cond: (((rec_isins.isin)::bpchar =
(attachment_isins.isin)::bpchar) AND (rec_isins.attachment =
attachment_isins.attachment))
Filter: (COALESCE(attachment_isins.attachment,
rec_isins.attachment) = 2698120)
-> Index Scan using rec_isin_pkey on rec_isins
(cost=0.00..229562.97 rows=8115133 width=17) (actual
time=0.141..18043.605 rows=8036226 loops=1)
-> Materialize (cost=71575.91..78318.19 rows=539383
width=17) (actual time=10181.074..14471.215 rows=539101 loops=1)
-> Sort (cost=71575.91..72924.36 rows=539383
width=17) (actual time=10181.064..13019.906 rows=539101 loops=1)
Sort Key: attachment_isins.isin,
attachment_isins.attachment
Sort Method: external merge Disk: 18936kB
-> Seq Scan on attachment_isins
(cost=0.00..13111.83 rows=539383 width=17) (actual time=0.036..912.963
rows=539101 loops=1)
Total runtime: 53937.213 ms
(12 rows)
These are the table definitions:
Table "ts_frontend.attachment_isins"
Column | Type | Modifiers
--------------+--------------------------------+-----------
attachment | integer | not null
isin | isin | not null
editor | name |
last_changed | timestamp(0) without time zone |
Indexes:
"attachment_isins_pkey" PRIMARY KEY, btree (attachment, isin)
"attachment_isins_attachment_idx" btree (attachment)
"attachment_isins_attachment_isin" btree (attachment, isin)
"attachment_isins_isin_idx" btree (isin)
Foreign-key constraints:
"attachment_isins_attachment_fkey" FOREIGN KEY (attachment)
REFERENCES ts_frontend.attachments(id) ON UPDATE CASCADE ON DELETE CASCADE
Table "ts_frontend.rec_isins"
Column | Type | Modifiers
------------+---------+-----------
attachment | integer | not null
isin | isin | not null
Indexes:
"rec_isin_pkey" PRIMARY KEY, btree (isin, attachment)
"idx_rec_isins_attachment" btree (attachment)
Foreign-key constraints:
"rec_isins_attachment_fkey" FOREIGN KEY (attachment) REFERENCES
ts_frontend.attachments(id) ON UPDATE CASCADE ON DELETE CASCADE
Thanks for any ideas!
Regards
Christian
P.S.: I think the full outer join is not what the developer really
wanted to do. Instead, he should have done a union (which is pretty
fast, by the way). However, I still want to understand why the query
plan of his query changed between both database releases.
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set work_mem=24000; before running the query. postgres is doing merge and sort on disc, that's always slow. is there an index on column isin ?
Grzegorz Jaśkiewicz wrote:
> set work_mem=24000; before running the query.
>
> postgres is doing merge and sort on disc, that's always slow.
>
Ok, but why is the plan different in 8.2? As you can see the same query
is really fast in 8.2, but slow in 8.3.
> is there an index on column isin ?
>
There is a separate index on the isin column of the attachment_isins
table (attachment_isins_isin_idx). The other table (rec_isins) has the
combination of attachment and isin as primary key which creates an
implicit index. Can this index be used for the single column isin? And
again: Why doesn't this matter in 8.2??
Regards,
Christian
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Financial IT and Consulting Fax: +49 551 489500-91
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2009/4/15 Christian Schröder <cs@deriva.de>: > Grzegorz Jaśkiewicz wrote: >> >> set work_mem=24000; before running the query. >> >> postgres is doing merge and sort on disc, that's always slow. >> > > Ok, but why is the plan different in 8.2? As you can see the same query is > really fast in 8.2, but slow in 8.3. Did that set help ? I think Tom will know more about it, but probably (and I am guessing here, to be honest) - Materialize plan wasn't either available, or didn't appear too be a planners favourite. on 8.2 the two loops instead were were much faster. Can you try increasing stat target to 100, vacuum analyze and see if different plan is choosen ? Again, I don't know at that point why is it so - just trying to suggests things that I would try . >> is there an index on column isin ? >> > > There is a separate index on the isin column of the attachment_isins table > (attachment_isins_isin_idx). The other table (rec_isins) has the combination > of attachment and isin as primary key which creates an implicit index. Can > this index be used for the single column isin? And again: Why doesn't this > matter in 8.2?? well, it is a different major release, and differences between 8.2->8.3 are vast. -- GJ
On Wed, 2009-04-15 at 14:04 +0200, Christian Schröder wrote: > Grzegorz Jaśkiewicz wrote: > > set work_mem=24000; before running the query. > > > > postgres is doing merge and sort on disc, that's always slow. > > > Ok, but why is the plan different in 8.2? As you can see the same query > is really fast in 8.2, but slow in 8.3. The cost of the query seems accurate, so the absence of attachment_isins_attachment_idx on the 8.3 plan looks to be the reason. There's no way it would choose to scan 8115133 rows on the pkey if the other index was available and usable. -- Simon Riggs www.2ndQuadrant.com PostgreSQL Training, Services and Support
On Wed, Apr 15, 2009 at 1:25 PM, Simon Riggs <simon@2ndquadrant.com> wrote: > The cost of the query seems accurate, so the absence of > attachment_isins_attachment_idx on the 8.3 plan looks to be the reason. > There's no way it would choose to scan 8115133 rows on the pkey if the > other index was available and usable. hance my question, if there's index on it in 8.3 version of db. -- GJ
=?ISO-8859-1?Q?Christian_Schr=F6der?= <cs@deriva.de> writes:
> This is the query:
> select isin from ts_frontend.attachment_isins full OUTER JOIN
> ts_frontend.rec_isins using (attachment,isin) WHERE attachment=2698120
> GROUP BY isin limit 1000;
Hmm. It seems 8.3 is failing to push the attachment=2698120 condition
down to the input relations. Not sure why. All that code got massively
rewritten in 8.3, but I thought it still understood about pushing
equalities through a full join ...
regards, tom lane
Grzegorz Jaśkiewicz wrote:
> On Wed, Apr 15, 2009 at 1:25 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
>
>> The cost of the query seems accurate, so the absence of
>> attachment_isins_attachment_idx on the 8.3 plan looks to be the reason.
>> There's no way it would choose to scan 8115133 rows on the pkey if the
>> other index was available and usable.
>>
>
> hance my question, if there's index on it in 8.3 version of db.
>
I added an index on this column, but it didn't change the query plan.
Stupid question: Do I have to analyze again or perform a reindex after
adding the index?
Regards,
Christian
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On Thu, Apr 16, 2009 at 01:31:45PM +0200, Christian Schröder wrote: > Stupid question: Do I have to analyze again or perform a reindex after > adding the index? No, it's a regression in PG's handling of outer joins---it used to realise that this was a possible optimisation, but now it doesn't. Tom Lane started discussion on -hackers about this issue: http://archives.postgresql.org/pgsql-hackers/2009-04/msg00849.php it looks as though performance in 8.3 is going to be bad until this behaviour is changed. A possible fix is to rewrite your query to work around the problem: SELECT isin FROM (SELECT * FROM ts_frontend.attachment_isins WHERE attachment = 2698120) a FULL OUTER JOIN (SELECT * FROM ts_frontend.rec_isins WHERE attachment = 2698120) USING (isin) GROUP BY isin LIMIT 1000; It looks as though what you're trying to do could also be expressed as: SELECT isin FROM ts_frontend.rec_isins WHERE attachment = 2698120 UNION SELECT isin FROM ts_frontend.attachment_isins WHERE attachment = 2698120; not sure if it's part of something larger so this may not be a useful transform. -- Sam http://samason.me.uk/
I wrote: > =?ISO-8859-1?Q?Christian_Schr=F6der?= <cs@deriva.de> writes: >> This is the query: >> select isin from ts_frontend.attachment_isins full OUTER JOIN >> ts_frontend.rec_isins using (attachment,isin) WHERE attachment=2698120 >> GROUP BY isin limit 1000; > Hmm. It seems 8.3 is failing to push the attachment=2698120 condition > down to the input relations. Not sure why. All that code got massively > rewritten in 8.3, but I thought it still understood about pushing > equalities through a full join ... I've applied a patch for this. It will be in 8.3.8, or if you're in a hurry you can grab it from our CVS server or here: http://archives.postgresql.org/message-id/20090416204228.579317540E2@cvs.postgresql.org regards, tom lane
On Thu, Apr 16, 2009 at 9:48 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote: > > I've applied a patch for this. It will be in 8.3.8, or if you're > in a hurry you can grab it from our CVS server or here: > > http://archives.postgresql.org/message-id/20090416204228.579317540E2@cvs.postgresql.org just out of curiosity - when was it introduced, ie - which version was the first affected ? We're still on 8.3.5 here. -- GJ
On Fri, Apr 17, 2009 at 9:22 AM, Grzegorz Jaśkiewicz <gryzman@gmail.com> wrote: > just out of curiosity - when was it introduced, ie - which version was > the first affected ? We're still on 8.3.5 here. (I had no idea release-notes have date), it got in by 8.3.4 (changed right after 8.3.3 was released). > > -- > GJ > -- GJ
Tom Lane wrote:
> I've applied a patch for this. It will be in 8.3.8, or if you're
> in a hurry you can grab it from our CVS server or here:
>
Thanks a lot for your effort and the quick response!
Regards,
Christian
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