Doc: add commentary about cowboy assignment of maintenance_work_mem.

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.

diff --git a/src/backend/commands/vacuumparallel.c b/src/backend/commands/vacuumparallel.c
index 49e605a6ffc55797226cc4a0606d82aa22482e62..dc3322c256b7db2e5e6535084f5d7e6ecc1e695a 100644 (file)
--- a/src/backend/commands/vacuumparallel.c
+++ b/src/backend/commands/vacuumparallel.c
@@ -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;