Re: Unexpectedly Long DELETE Wait

Volkan YAZICI wrote:
> Hi,
>
> Below command has been running since ~700 minutes in one of our
> PostgreSQL servers.
>
>   DELETE FROM mugpsreglog
>         WHERE NOT EXISTS (SELECT 1
>                             FROM mueventlog
>                            WHERE mueventlog.eventlogid = mugpsreglog.eventlogid);
>
>    Seq Scan on mugpsreglog  (cost=0.00..57184031821394.73 rows=6590986 width=6)
>      Filter: (NOT (subplan))
>         SubPlan
>              ->  Seq Scan on mueventlog  (cost=0.00..4338048.00 rows=1 width=0)
>              Filter: (eventlogid = $0)

Ouch - look at the estimated cost on that!

> And there isn't any constraints (FK/PK), triggers, indexes, etc. on any
> of the tables. (We're in the phase of a migration, many DELETE commands
> similar to above gets executed to relax constraints will be introduced.)

Well there you go. Add an index on eventlogid for mugpsreglog.

Alternatively, if you increased your work_mem that might help. Try SET
work_mem='64MB' (or even higher) before running the explain and see if
it tries a materialize. For situations like this where you're doing big
one-off queries you can afford to increase resource limits.

--
   Richard Huxton
   Archonet Ltd