Date: Fri, 22 Feb 2019 09:54:15 -0600
From: Justin Pryzby <pryzby@telsasoft.com>
To: kimaidou <kimaidou@gmail.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Aggregate and many LEFT JOIN
Message-ID: <20190222155415.GE28750@telsasoft.com>
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On Fri, Feb 22, 2019 at 04:14:05PM +0100, kimaidou wrote:
> Explain shows that the GROUP AGGREGATE and needed sort kill the performance.
> Do you have any hint how to optimize this ?
> https://explain.depesz.com/s/6nf

This is writing 2GB tempfile, perhaps the query would benefit from larger
work_mem:
|Sort (cost=3,014,498.66..3,016,923.15 rows=969,796 width=1,818) (actual time=21,745.193..22,446.561 rows=1,212,419 loops=1)
|   Sort Method: external sort Disk: 1782200kB
|   Buffers: shared hit=5882951, temp read=230958 written=230958

This is apparently joining without indices:
|Nested Loop Left Join (cost=1.76..360,977.37 rows=321,583 width=1,404) (actual time=0.080..1,953.007 rows=321,849 loops=1)
|   Join Filter: (tgc1.groupe_nom = t.group1_inpn)
|   Rows Removed by Join Filter: 965547
|   Buffers: shared hit=1486327

This perhaps should have an index on tgc2.groupe_type ?
|Index Scan using t_group_categorie_pkey on taxon.t_group_categorie tgc2 (cost=0.14..0.42 rows=1 width=28) (actual time=0.002..0.002 rows=1 loops=321,849)
|   Index Cond: (tgc2.groupe_nom = t.group2_inpn)
|   Filter: (tgc2.groupe_type = 'group2_inpn'::text)
|   Buffers: shared hit=643687

This would perhaps benefit from an index on tv.cd_ref ?
|Index Scan using taxref_consolide_non_filtre_cd_nom_idx on taxon.taxref_consolide_non_filtre tv (cost=0.42..0.63 rows=1 width=94) (actual time=0.002..0.002 rows=1 loops=690,785)
|   Index Cond: (tv.cd_nom = t.cd_ref)
|   Filter: (tv.cd_nom = tv.cd_ref)
|   Buffers: shared hit=2764875

I don't think it's causing a significant fraction of the issue, but for some
reason this is overestimating rowcount by 2000.  Do you need to VACUUM ANALYZE
the table ?
|Seq Scan on occtax.personne p_1 (cost=0.00..78.04 ROWS=2,204 width=56) (actual time=0.011..0.011 ROWS=1 loops=1)

Justin