Avoid passing zero as a value for vacuum_cost_limit, because it's not a valid

value for the vacuum code.  Instead, make zero signify getting the value from a
higher level configuration facility, just like -1 in the original coding.  We
still document that -1 is the value that disables the feature, to avoid
confusing the user unnecessarily.

Reported by Galy Lee in <200705310914.l4V9E6JA094603@wwwmaster.postgresql.org>;
per subsequent discussion.

doc/src/sgml/catalogs.sgml

@@ -1,4 +1,4 @@
-<!-- $PostgreSQL: pgsql/doc/src/sgml/catalogs.sgml,v 2.153 2007/06/05 21:31:03 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/catalogs.sgml,v 2.154 2007/06/08 21:09:49 alvherre Exp $ -->
 <!--
  Documentation of the system catalogs, directed toward PostgreSQL developers
  -->
@@ -1339,7 +1339,7 @@
    be used for this particular value.  Observe that the
    <structfield>vac_cost_delay</> variable inherits its default value from the
    <xref linkend="guc-autovacuum-vacuum-cost-delay"> configuration parameter,
-   or from <varname>vacuum_cost_delay</> if the former is set to a negative
+   or from <xref linkend="guc-vacuum-cost-delay"> if the former is set to a negative
    value.  The same applies to <structfield>vac_cost_limit</>.
    Also, autovacuum will ignore attempts to set a per-table
    <structfield>freeze_max_age</> larger than the system-wide setting (it can only be set

src/backend/postmaster/autovacuum.c

@@ -10,7 +10,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/postmaster/autovacuum.c,v 1.47 2007/05/30 20:11:57 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/postmaster/autovacuum.c,v 1.48 2007/06/08 21:09:49 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1548,7 +1548,11 @@ static void
 autovac_balance_cost(void)
 {
 	WorkerInfo	worker;
-	int         vac_cost_limit = (autovacuum_vac_cost_limit >= 0 ?
+	/*
+	 * note: in cost_limit, zero also means use value from elsewhere, because
+	 * zero is not a valid value.
+	 */
+	int         vac_cost_limit = (autovacuum_vac_cost_limit > 0 ?
 								  autovacuum_vac_cost_limit : VacuumCostLimit);
 	int         vac_cost_delay = (autovacuum_vac_cost_delay >= 0 ?
 								  autovacuum_vac_cost_delay : VacuumCostDelay);
@@ -2140,12 +2144,14 @@ table_recheck_autovac(Oid relid)
 		 * there is a tuple in pg_autovacuum, use it; else, use the GUC
 		 * defaults.  Note that the fields may contain "-1" (or indeed any
 		 * negative value), which means use the GUC defaults for each setting.
+		 * In cost_limit, the value 0 also means to use the value from
+		 * elsewhere.
 		 */
 		if (avForm != NULL)
 		{
-			vac_cost_limit = (avForm->vac_cost_limit >= 0) ?
+			vac_cost_limit = (avForm->vac_cost_limit > 0) ?
 				avForm->vac_cost_limit :
-				((autovacuum_vac_cost_limit >= 0) ?
+				((autovacuum_vac_cost_limit > 0) ?
 				 autovacuum_vac_cost_limit : VacuumCostLimit);
 
 			vac_cost_delay = (avForm->vac_cost_delay >= 0) ?
@@ -2158,7 +2164,7 @@ table_recheck_autovac(Oid relid)
 		}
 		else
 		{
-			vac_cost_limit = (autovacuum_vac_cost_limit >= 0) ?
+			vac_cost_limit = (autovacuum_vac_cost_limit > 0) ?
 				autovacuum_vac_cost_limit : VacuumCostLimit;
 
 			vac_cost_delay = (autovacuum_vac_cost_delay >= 0) ?