Re: wildcard text filter switched to boolean column, performance is way worse

Hello,
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> From: pgsql-performance-owner(at)postgresql(dot)org [mailto:pgsql-performance-owner(at)postgresql(dot)org] On Behalf Of Mike Broers
> Sent: Dienstag, 7. Juli 2015 18:28
> To: Tom Lane
> Cc: pgsql-performance(at)postgresql(dot)org
> Subject: Re: [PERFORM] wildcard text filter switched to boolean column, performance is way worse
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> After bumping up work_mem from 12MB to 25MB that last materialize is indeed hashing and this cut the query time by about 60%. Thanks, this was very helpful and gives me something else to look for when troubleshooting explains.
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> On Tue, Jul 7, 2015 at 11:10 AM, Mike Broers <mbroers(at)gmail(dot)com> wrote:
> Thanks, very informative! I'll experiment with work_mem settings and report back.
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> On Tue, Jul 7, 2015 at 11:02 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Mike Broers <mbroers(at)gmail(dot)com> writes:
> > I had a query that was filtering with a wildcard search of a text field for
> > %SUCCESS%. The query took about 5 seconds and was running often so I wanted
> > to improve it. I suggested that the engineers include a new boolean column
> > for successful status. They implemented the requested field, but the query
> > that filters on that new column runs very long (i kill it after letting it
> > run for about an hour). Can someone help me understand why that is the
> > case and how to resolve it?
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> It's hashing the subplan output in the first case and not the second:
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> > Seq Scan on lead (cost=130951.81..158059.21 rows=139957 width=369) (actual
> > time=4699.619..4699.869 rows=1 loops=1)
> > Filter: ((NOT (hashed SubPlan 1)) AND (("ReferenceNumber")::text <>
> > ''::text) AND ((NOT (hashed SubPlan 2)) OR (NOT (hashed SubPlan 3))))
> ^^^^^^^^^^^^^^^^
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> > Seq Scan on lead (cost=85775.78..9005687281.12 rows=139957 width=369)
> > Filter: ((NOT (hashed SubPlan 1)) AND (("ReferenceNumber")::text <>
> > ''::text) AND ((NOT (hashed SubPlan 2)) OR (NOT (SubPlan 3))))
> ^^^^^^^^^
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> Presumably, the new more-accurate rows count causes the planner to realize
> that the hash table will exceed work_mem so it doesn't choose to hash ...
> but for your situation, you'd rather it did, because what you're getting
> instead is a Materialize node that spills to disk (again, because the data
> involved exceeds work_mem) and that's a killer for this query. You should
> be able to get back the old behavior if you raise work_mem enough.
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> Another idea you might think about is changing the OR'd IN conditions
> to a single IN over a UNION ALL of the subselects. I'm not really sure if
> that would produce a better plan, but it's worth trying if it wouldn't
> require too much app-side contortion.

Hello,
you might try to use a CTE to first collect the IDs to exclude, and join them to your main table.
This should result in an anti join pattern.

Something like:

WITH IDS as (
SELECT U1."lead_id" AS "lead_id"
FROM "event" U1
WHERE U1."event_type" ='type_1'
UNION (
SELECT U1."lead_id" AS "lead_id"
FROM "event" U1
WHERE U1."event_type" = 'type_2'
INTERSECT
SELECT U1."lead_id" AS "lead_id"
FROM "event" U1
WHERE successful
)
)
SELECT * FROM lead LEFT OUTER JOIN IDS ON (lead.id=IDS.lead_id)
WHERE IDS.lead_id IS NULL;

regards,

Marc Mamin

> regards, tom lane
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