On Tue, Feb 28, 2023 at 07:06:43PM +0100, Jehan-Guillaume de Rorthais wrote:
> Hello all,
>
> A customer is facing out of memory query which looks similar to this situation:
>
> https://www.postgresql.org/message-id/flat/12064.1555298699%40sss.pgh.pa.us#eb519865575bbc549007878a5fb7219b
>
> This PostgreSQL version is 11.18. Some settings:
hash joins could exceed work_mem until v13:
|Allow hash aggregation to use disk storage for large aggregation result
|sets (Jeff Davis)
|
|Previously, hash aggregation was avoided if it was expected to use more
|than work_mem memory. Now, a hash aggregation plan can be chosen despite
|that. The hash table will be spilled to disk if it exceeds work_mem
|times hash_mem_multiplier.
|
|This behavior is normally preferable to the old behavior, in which once
|hash aggregation had been chosen, the hash table would be kept in memory
|no matter how large it got — which could be very large if the planner
|had misestimated. If necessary, behavior similar to that can be obtained
|by increasing hash_mem_multiplier.
> https://explain.depesz.com/s/sGOH
This shows multiple plan nodes underestimating the row counts by factors
of ~50,000, which could lead to the issue fixed in v13.
I think you should try to improve the estimates, which might improve
other queries in addition to this one, in addition to maybe avoiding the
issue with joins.
> The customer is aware he should rewrite this query to optimize it, but it's a
> long time process he can not start immediately. To make it run in the meantime,
> he actually removed the top CTE to a dedicated table.
Is the table analyzed ?
> Is it usual a backend is requesting such large memory size (13 GB) and
> actually use less of 60% of it (7.7GB of RSS)?
It's possible it's "using less" simply because it's not available. Is
the process swapping ?
--
Justin