From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Josef Machytka <josef(dot)machytka(at)gmail(dot)com> |
Cc: | fabriziomello(at)gmail(dot)com, pgsql-bugs(at)postgresql(dot)org |
Subject: | Re: BUG #14845: postgres_fdw and mysql_fdw can cause crash of instance |
Date: | 2017-10-06 17:01:26 |
Message-ID: | 9948.1507309286@sss.pgh.pa.us |
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Lists: | pgsql-bugs |
Josef Machytka <josef(dot)machytka(at)gmail(dot)com> writes:
> In this case which went wrong I used postgres_fdw to compare data on local
> and remote database using "select all from remote except select all from
> local".
I think my theory B probably applies then: without use_remote_estimate,
the planner would not guess that the remote table is huge, and that could
well allow it to pick a hashed EXCEPT implementation --- which will then
try to collect the entire remote table into an in-memory hash table.
I don't know whether mysql_fdw has a comparable switch, but in any case
if it wasn't turned on then the same failure mode could apply to that FDW.
I wonder if we should rethink the default setting of use_remote_estimate
being "off". Particularly as we keep on allowing postgres_fdw to consider
more different plan types, it seems like the potential for stupid plan
choices is getting worse all the time.
regards, tom lane
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