From: Tom Lane <tgl@sss.pgh.pa.us>
To: Sergey Koposov <Sergey.Koposov@ed.ac.uk>
Cc: lepihov@gmail.com <lepihov@gmail.com>
Cc: pgsql-bugs@lists.postgresql.org <pgsql-bugs@lists.postgresql.org>
Subject: Re: BUG #18909: Query creates millions of temporary files and stalls
Date: Sat, 03 May 2025 18:03:26 -0400
Message-ID: <2216372.1746309806@sss.pgh.pa.us> (raw)
In-Reply-To: <0ce957be37434cfca4820d1e89a70cb2b456d3db.camel@ed.ac.uk>
References: <18909-e5e1b702c9441b8a@postgresql.org>
<2051617.1746223978@sss.pgh.pa.us>
<beb051161520fc48248c2f90582eb15c6a71d239.camel@ed.ac.uk>
<5e98fba645682c40bf5f88ef858963ade9681956.camel@ed.ac.uk>
<2177960.1746289626@sss.pgh.pa.us>
<80bab58be182e80e73b5d5f71664ede9ce58f957.camel@ed.ac.uk>
<8fc51cd7-115a-425a-828e-c99defb5afef@gmail.com>
<ba927eb7126844c1b964e75cadb774488794dfde.camel@ed.ac.uk>
<0ce957be37434cfca4820d1e89a70cb2b456d3db.camel@ed.ac.uk>
Sergey Koposov <Sergey.Koposov@ed.ac.uk> writes:
> -> Parallel Hash Left Join (cost=14442102.04..22124798.60 rows=16367717 width=60) (actual time=209805.943..273008.489 rows=13102859 loops=5)
> Hash Cond: (d.objid = gaps1.original_ext_source_id)
> -> Parallel Seq Scan on disk_sample1 d (cost=0.00..1218371.17 rows=16367717 width=60) (actual time=37.353..25185.340 rows=13095751 loops=5)
> -> Parallel Hash (cost=10307380.24..10307380.24 rows=237862624 width=16) (actual time=169633.067..169633.068 rows=190290095 loops=5)
> Buckets: 67108864 Batches: 32 Memory Usage: 1919904kB
> -> Parallel Seq Scan on panstarrs1bestneighbour gaps1 (cost=0.00..10307380.24 rows=237862624 width=16) (actual time=132.295..117548.803
> rows=190290095 loops=5)
Hm, interesting. The number of batches stayed sane here (32), whereas
it went crazy in the other run. I wonder if there's something
nondeterministic about that choice in a parallel hash join.
> I don't think I know how to see the plan of the declare cursor query.
EXPLAIN DECLARE c CURSOR FOR SELECT ...
regards, tom lane
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To: pgsql-bugs@postgresql.org
Cc: tgl@sss.pgh.pa.us, Sergey.Koposov@ed.ac.uk, lepihov@gmail.com, pgsql-bugs@lists.postgresql.org
Subject: Re: BUG #18909: Query creates millions of temporary files and stalls
In-Reply-To: <2216372.1746309806@sss.pgh.pa.us>
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