Re: Primary key index suddenly became very slow

El lun, 08-02-2016 a las 10:04 +0000, Gustav Karlsson escribió:
> Additional information:
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> The problematic row has likely received many hot updates (100k+).
> Could this be a likely explanation for the high execution time?
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Could you check if autovacuum is doing its job with this query:
select * from pg_stat_user_tables where relname='konto' , is it last_autovaccum and last_autoanalyze recent ?
if you don't reduce n_dead_tup in a short time after the bulk process of hot update, it will be a explanation, and also a "idle in transaction" connection could cause it.
This link: https://brandur.org/postgres-queues could help you.

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> Regards,
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> Gustav
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> > On Feb 8, 2016, at 10:45 AM, Gustav Karlsson <gustav(dot)karlsson(at)bekk(dot)no> wrote:
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> > Hi,
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> > Question:
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> > What may cause a primary key index to suddenly become very slow? Index scan for single row taking 2-3 seconds. A manual vacuum resolved the problem.
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> > Background:
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> > We have a simple table ‘KONTO’ with about 600k rows.
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> > Column | Type | Modifiers
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> > ------------------------------+-----------------------------+---------------
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> > id | bigint | not null
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> > ...
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> > Indexes:
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> > "konto_pk" PRIMARY KEY, btree (id)
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> > ...
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> > Over the weekend we experienced that lookups using the primary key index (‘konto_pk’) became very slow, in the region 2-3s for fetching a single record:
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> > QUERY PLAN
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> > Index Scan using konto_pk on konto (cost=0.42..6.44 rows=1 width=164) (actual time=0.052..2094.549 rows=1 loops=1)
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> > Index Cond: (id = 2121172829)
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> > Planning time: 0.376 ms
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> > Execution time: 2094.585 ms
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> > After a manual Vacuum the execution time is OK:
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> > QUERY PLAN
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> > Index Scan using konto_pk on konto (cost=0.42..6.44 rows=1 width=164) (actual time=0.037..2.876 rows=1 loops=1)
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> > Index Cond: (id = 2121172829)
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> > Planning time: 0.793 ms
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> > Execution time: 2.971 ms
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> > So things are working OK again, but we would like to know what may cause such a degradation of the index scan, to avoid this happening again? (We are using Postgresql version 9.4.4)
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> > Regards,
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> > Gustav
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