Re: Temp files for simple queries

On Tue, Jun 22, 2021 at 01:51:55PM +0200, Radoulov, Dimitre wrote:
> Just to clarify: the size of the temp file is 7MB and with work_mem of 4MB
> it clearly doesn't fit.
>
> But why an "order by" of a 440kB of data would require 7MB.
>
> One possible reason is that that the number of records in that table varies
> significantly during the day,

That would be the logical explanation. You could configure auto_explain to
make sure of that (https://www.postgresql.org/docs/current/auto-explain.html)

> but - if that's the case - I would expect that
> the table would be much bigger than 440kB (size taken using \dt+).

Not necessarily. autovacuum can truncate the file if all the trailing blocks
are empty (and if it can acquire an exclusive lock fast enough), so if some
client inserts a lot of rows, process them and remove them all, and later on
slowly start to insert new rows you will get that behavior.