On Wed, 4 Jun 2025, Frédéric Yhuel wrote:
On 6/3/25 17:34, Dimitrios Apostolou wrote:
The backend process for each of the above ALTER TABLE commands, does not
parallelize the foreign key checks for the different partitions. I
know, because in the logs I see gigabytes of temporary files being
written, with the CONTEXT showing queries issued incrementally on
all the different partitions:
:LOG: temporary file: path "pg_tblspc/16390/PG_17_202406281/
pgsql_tmp/pgsql_tmp3363462.579", size 1073741824
:CONTEXT: SQL statement "SELECT fk."columnX" FROM ONLY
"public"."table_partition_214" fk
LEFT OUTER JOIN ONLY "public"."another_table" pk
ON ( pk."columnX" OPERATOR(pg_catalog.=) fk."columnX")
WHERE pk."columnX" IS NULL AND (fk."columnX" IS NOT NULL)"
Why can't the backend issue these queries in parallel workers?
This has been discussed here:
https://www.postgresql.org/message-id/flat/0d21e3b4-dcde-290c-875e-6ed5013e8e52%40dalibo.com
Perhaps we should exhume this patch, but I believe the optimal strategy is to
perform a VACUUM between the data and post-data to build the visibility map.
The anti-join can then use an efficient index-only scan.
Thanks for pointing to this patch.
Since I run each of the pg_restore sections separately, I will try to
manually do a VACUUM after the "data" and before the "post-data" section.
In general I have noticed most operations are slower after a succesful
pg_restore until VACUUM is complete, which is unfortunate as the database
is huge and it takes days to run. Something I have on my list to try, is
whether a COPY FREEZE would alleviate all this trouble, since all tuples
are immediately visible then. Maybe a patch for a new pg_restore
option --freeze is a better solution. Are my assumptions right?
Thanks,
Dimitris
