Re: Temp files for simple queries

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

Regards
Dimitre

On 22/06/2021 13.39, Radoulov, Dimitre wrote:
>
> Hello all,
>
> We have a small Google Cloud SQL PostgreSQL 11 instance.
> The instance is configured with 8G of memory and 4 vCPUs, work_mem is
> 4MB.
>
> I was wondering why the instance uses disk temp files for such a small
> amount of data (see size in the log below)?
>
> I suppose that it's not only the work_mem limit that could trigger
> disk temp file creation or the reported SQL statement is not the only
> one that contributes to it:
>
> I 2021-06-22T09:12:59.164913Z 2021-06-22 09:12:59.163 UTC [1957798]:
> [2-1] db=<db>,user=<user> STATEMENT: SELECT c1, c2 FROM schema1.t1
> WHERE c1 >= $1 ORDER BY c1
> I 2021-06-22T09:12:59.164379Z 2021-06-22 09:12:59.163 UTC [1957798]:
> [1-1] db=<db>,user=<user> LOG: temporary file: path
> "base/pgsql_tmp/pgsql_tmp1957798.0", size 7380992
>
> The t1 table (table and column names have been masked for privacy)
> occupies only  440 kB and has 160 records.
>
>
> Best regards
> Dimitre
>
>
>