Lower minimum maintenance_work_mem to 64kB

Since the introduction of TID store, vacuum uses far less memory in
the common case than in versions 16 and earlier. Invoking multiple
rounds of index vacuuming in turn requires a much larger table. It'd
be a good idea anyway to cover this case in regression testing, and a
lower limit is less painful for slow buildfarm animals. The reason to
do it now is to re-enable coverage of the bugfix in commit 83c39a1f7f.

For consistency, give autovacuum_work_mem the same treatment.

Suggested by Andres Freund
Tested by Melanie Plageman
Backpatch to v17, where TID store was introduced

Discussion: https://postgr.es/m/20240516205458.ohvlzis5b5tvejru@awork3.anarazel.de
Discussion: https://postgr.es/m/20240722164745.fvaoh6g6zprisqgp%40awork3.anarazel.de

src/backend/postmaster/autovacuum.c

@@ -3339,12 +3339,12 @@ check_autovacuum_work_mem(int *newval, void **extra, GucSource source)
 		return true;
 
 	/*
-	 * We clamp manually-set values to at least 1MB.  Since
+	 * We clamp manually-set values to at least 64kB.  Since
 	 * maintenance_work_mem is always set to at least this value, do the same
 	 * here.
 	 */
-	if (*newval < 1024)
-		*newval = 1024;
+	if (*newval < 64)
+		*newval = 64;
 
 	return true;
 }

src/backend/utils/misc/guc_tables.c

@@ -2510,6 +2510,11 @@ struct config_int ConfigureNamesInt[] =
 		NULL, NULL, NULL
 	},
 
+	/*
+	 * Dynamic shared memory has a higher overhead than local memory contexts,
+	 * so when testing low-memory scenarios that could use shared memory, the
+	 * recommended minimum is 1MB.
+	 */
 	{
 		{"maintenance_work_mem", PGC_USERSET, RESOURCES_MEM,
 			gettext_noop("Sets the maximum memory to be used for maintenance operations."),
@@ -2517,7 +2522,7 @@ struct config_int ConfigureNamesInt[] =
 			GUC_UNIT_KB
 		},
 		&maintenance_work_mem,
-		65536, 1024, MAX_KILOBYTES,
+		65536, 64, MAX_KILOBYTES,
 		NULL, NULL, NULL
 	},
 

src/backend/utils/misc/postgresql.conf.sample

@@ -139,8 +139,8 @@
 # you actively intend to use prepared transactions.
 #work_mem = 4MB				# min 64kB
 #hash_mem_multiplier = 2.0		# 1-1000.0 multiplier on hash table work_mem
-#maintenance_work_mem = 64MB		# min 1MB
-#autovacuum_work_mem = -1		# min 1MB, or -1 to use maintenance_work_mem
+#maintenance_work_mem = 64MB		# min 64kB
+#autovacuum_work_mem = -1		# min 64kB, or -1 to use maintenance_work_mem
 #logical_decoding_work_mem = 64MB	# min 64kB
 #max_stack_depth = 2MB			# min 100kB
 #shared_memory_type = mmap		# the default is the first option