Re: Slow update on column that is part of exclusion constraint - Mailing list pgsql-performance

> On 14 Apr 2016, at 07:17, Adam Brusselback <adambrusselback@gmail.com> wrote:
>
> So fair enough, it does seem to be related to the lookup rather than maintenance on the index. I was misguided in my
initialassumption.   
>
> Spent quite a bit of time trying to come up with a self contained test, and it seems like I can't make it choose the
GiSTindex unless I remove the regular btree index in my test case, though the opposite is true for my table in
production. Not really sure what that means as far as what I need to do though. I've tried a vacuum full, analyze,
rebuildindex, drop and re-add the constraint... It still uses that GiST index for this query. 
>
> Hell, a sequential scan is a ton faster even.
>

As i understand it, postgres needs a way to find rows for update.
In explain analyze you provided, we see that it chose gist index for that.
And that is a poor chose. I think you need a proper btree index for update
query to work properly fast. Like index on (product_id, company_id, date_range) WHERE
upper(price_generated_test.active_range)IS NULL.  



> On Wed, Apr 13, 2016 at 2:54 PM, Evgeniy Shishkin <itparanoia@gmail.com> wrote:
>
> > On 13 Apr 2016, at 20:14, Adam Brusselback <adambrusselback@gmail.com> wrote:
> >
> > Sorry, brain stopped working and I forgot to include the normal info.
> >
> > Postgres version: 9.5.1
> > Hardware: 2 core, 4gb Digital Ocean virtual server
> > OS: Debian
> >
> > explain analyze for an example update:
> > 'Update on price_generated  (cost=32.45..644.83 rows=1 width=157) (actual time=29329.614..29329.614 rows=0
loops=1)'
> > '  ->  Nested Loop  (cost=32.45..644.83 rows=1 width=157) (actual time=29329.608..29329.608 rows=0 loops=1)'
> > '        ->  HashAggregate  (cost=32.04..34.35 rows=231 width=52) (actual time=1.137..2.090 rows=231 loops=1)'
> > '              Group Key: pti.product_id, pti.company_id, pti.date_range'
> > '              ->  Seq Scan on _prices_to_insert pti  (cost=0.00..30.31 rows=231 width=52) (actual
time=0.060..0.678rows=231 loops=1)' 
> > '        ->  Index Scan using price_generated_company_product_date_active_excl on price_generated  (cost=0.41..2.63
rows=1width=151) (actual time=126.949..126.949 rows=0 loops=231)' 
> > '              Index Cond: (date_range = pti.date_range)'
> > '              Filter: ((upper(active_range) IS NULL) AND (pti.product_id = product_id) AND (pti.company_id =
company_id))'
> > '              Rows Removed by Filter: 29460'
> > 'Planning time: 3.134 ms'
> > 'Execution time: 29406.717 ms'
>
> Well, you see execution time of 30 seconds because there are 231 index lookups,
> each taking 126 ms.
>
> And that lookup is slow because of
> Filter: ((upper(active_range) IS NULL) AND (pti.product_id = product_id) AND (pti.company_id = company_id))'
>
> Can you provide self-containing example of update?
> I don't see there (upper(active_range) IS NULL condition is coming from.
>
> <excl constraint test case.sql>
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