diff --git a/src/backend/commands/vacuumparallel.c b/src/backend/commands/vacuumparallel.c
index 49e605a6ffc..dc3322c256b 100644
--- 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;