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Doc: add commentary about cowboy assignment of maintenance_work_mem.

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Whilst working on commit 041e8b95b I happened to notice that
parallel_vacuum_main() assigns directly to the maintenance_work_mem
GUC.  This is definitely not per project conventions, so I tried to
fix it to use SetConfigOption().  But that fails with "parameter
cannot be set during a parallel operation".  It doesn't seem worth
working on a cleaner answer, at least not till we have a few more
instances of similar problems.  But add some commentary, just so
nobody gets the idea that this is an approved way to set a GUC.
This commit is contained in:
Tom Lane 2025-01-31 15:17:15 -05:00
parent d4c3a6b8ad
commit 53a4936505
1 changed files with 7 additions and 0 deletions
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7
src/backend/commands/vacuumparallel.c
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@ -1032,6 +1032,13 @@ parallel_vacuum_main(dsm_segment *seg, shm_toc *toc)
vac_open_indexes(rel, RowExclusiveLock, &nindexes, &indrels);
Assert(nindexes > 0);
/*
* Apply the desired value of maintenance_work_mem within this process.
* Really we should use SetConfigOption() to change a GUC, but since we're
* already in parallel mode guc.c would complain about that. Fortunately,
* by the same token guc.c will not let any user-defined code change it.
* So just avert your eyes while we do this:
*/
if (shared->maintenance_work_mem_worker > 0)
maintenance_work_mem = shared->maintenance_work_mem_worker;