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If I see something like:

db=> SELECT datname,pid,state, (now() - query_start)::interval, query FROM pg_stat_activity where state != 'idle' and query like 'autovacuum%';
 datname |  pid  | state  |    interval     |                        query                        
---------+-------+--------+-----------------+-----------------------------------------------------
 theseus | 21936 | active | 00:00:00.940774 | autovacuum: VACUUM ANALYZE pg_catalog.pg_constraint
 theseus | 16775 | active | 00:33:57.896069 | autovacuum: VACUUM pg_toast.pg_toast_1234

How should I interpret xact_start vis a vis the vacuum_cost_limit and autovacuum_vacuum_cost_delay? In other words is xact_start the time at which the thread woke up again to do another chunk of vacuum_cost_limit work, or does it represent many chunks?

And a follow-up: my vacuum_cost_limit setting is the default of 200 (very conservative); if we're seeing a very long "transaction" interval and that interval should be measuring a very small amount of work, what could explain the long runtime? It's my understanding that autovacuum doesn't block on other activity.

  • you're mixing query_start in the example ans xact_start in the text, but for autovacuum I would expect the same value for both. – Jasen May 19 '18 at 3:04
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The xact_start field should be before or equal to query_start, covering the entire duration of the vacuuming of the table.

Your vacuum is slow because your setting of vacuum_cost_limit is very conservative. It is throttling itself, not blocking on other things.

  • I don't doubt it (it seems most likely to me too), but do you have a source or can you explain how you came to that conclusion? – jberryman May 17 '18 at 14:37
  • There are cases where autovac will block trying to get a clean-up pin on a buffer, but it is very rare for this to be frequent enough or long enough to explain problems. The default conservatism of vacuum throttling settings is not rare, so it is a good bet. I don't know of a good way to get definitive evidence, though, other than running strace on the vacuum worker and seeing what it is doing. – jjanes May 20 '18 at 21:27

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