guc_parallel_index_scan_v1.patch

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index dc63d7d..b4b7f0b 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -3835,20 +3835,34 @@ ANY <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="
       </listitem>
      </varlistentry>
 
-     <varlistentry id="guc-min-parallel-relation-size" xreflabel="min_parallel_relation_size">
-      <term><varname>min_parallel_relation_size</varname> (<type>integer</type>)
+     <varlistentry id="guc-min-parallel-table-scan-size" xreflabel="min_parallel_table_scan_size">
+      <term><varname>min_parallel_table_scan_size</varname> (<type>integer</type>)
       <indexterm>
-       <primary><varname>min_parallel_relation_size</> configuration parameter</primary>
+       <primary><varname>min_parallel_table_scan_size</> configuration parameter</primary>
       </indexterm>
       </term>
       <listitem>
        <para>
-        Sets the minimum size of relations to be considered for parallel scan.
+        Sets the minimum size of tables to be considered for parallel scan.
         The default is 8 megabytes (<literal>8MB</>).
        </para>
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-min-parallel-index-scan-size" xreflabel="min_parallel_index_scan_size">
+      <term><varname>min_parallel_index_scan_size</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>min_parallel_index_scan_size</> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Sets the minimum size of indexes to be considered for parallel scan.
+        The default is 512 kilobytes (<literal>512kB</>).
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-effective-cache-size" xreflabel="effective_cache_size">
       <term><varname>effective_cache_size</varname> (<type>integer</type>)
       <indexterm>
diff --git a/src/backend/optimizer/path/allpaths.c b/src/backend/optimizer/path/allpaths.c
index 5c18987..85505c5 100644
--- a/src/backend/optimizer/path/allpaths.c
+++ b/src/backend/optimizer/path/allpaths.c
@@ -57,7 +57,8 @@ typedef struct pushdown_safety_info
 /* These parameters are set by GUC */
 bool		enable_geqo = false;	/* just in case GUC doesn't set it */
 int			geqo_threshold;
-int			min_parallel_relation_size;
+int			min_parallel_table_scan_size;
+int			min_parallel_index_scan_size;
 
 /* Hook for plugins to get control in set_rel_pathlist() */
 set_rel_pathlist_hook_type set_rel_pathlist_hook = NULL;
@@ -126,7 +127,8 @@ static void subquery_push_qual(Query *subquery,
 static void recurse_push_qual(Node *setOp, Query *topquery,
 				  RangeTblEntry *rte, Index rti, Node *qual);
 static void remove_unused_subquery_outputs(Query *subquery, RelOptInfo *rel);
-static int	compute_parallel_worker(RelOptInfo *rel, BlockNumber pages);
+static int compute_parallel_worker(RelOptInfo *rel, BlockNumber heap_pages,
+						BlockNumber index_pages);
 
 
 /*
@@ -679,7 +681,7 @@ create_plain_partial_paths(PlannerInfo *root, RelOptInfo *rel)
 {
 	int			parallel_workers;
 
-	parallel_workers = compute_parallel_worker(rel, rel->pages);
+	parallel_workers = compute_parallel_worker(rel, rel->pages, 0);
 
 	/* If any limit was set to zero, the user doesn't want a parallel scan. */
 	if (parallel_workers <= 0)
@@ -2876,13 +2878,20 @@ remove_unused_subquery_outputs(Query *subquery, RelOptInfo *rel)
 
 /*
  * Compute the number of parallel workers that should be used to scan a
- * relation.  "pages" is the number of pages from the relation that we
- * expect to scan.
+ * relation.  We compute the parallel workers based on the size of the heap to
+ * be scanned and the size of the index to be scanned, then choose a minimum
+ * of those.
+ *
+ * "heap_pages" is the number of pages from the table that we expect to scan.
+ * "index_pages" is the number of pages from the index that we expect to scan.
  */
 static int
-compute_parallel_worker(RelOptInfo *rel, BlockNumber pages)
+compute_parallel_worker(RelOptInfo *rel, BlockNumber heap_pages,
+						BlockNumber index_pages)
 {
-	int			parallel_workers;
+	int			parallel_workers = 0;
+	int			heap_parallel_workers = 1;
+	int			index_parallel_workers = 1;
 
 	/*
 	 * If the user has set the parallel_workers reloption, use that; otherwise
@@ -2892,7 +2901,8 @@ compute_parallel_worker(RelOptInfo *rel, BlockNumber pages)
 		parallel_workers = rel->rel_parallel_workers;
 	else
 	{
-		int			parallel_threshold;
+		int			heap_parallel_threshold;
+		int			index_parallel_threshold;
 
 		/*
 		 * If this relation is too small to be worth a parallel scan, just
@@ -2901,25 +2911,48 @@ compute_parallel_worker(RelOptInfo *rel, BlockNumber pages)
 		 * might not be worthwhile just for this relation, but when combined
 		 * with all of its inheritance siblings it may well pay off.
 		 */
-		if (pages < (BlockNumber) min_parallel_relation_size &&
+		if (heap_pages < (BlockNumber) min_parallel_table_scan_size &&
+			index_pages < (BlockNumber) min_parallel_index_scan_size &&
 			rel->reloptkind == RELOPT_BASEREL)
 			return 0;
 
-		/*
-		 * Select the number of workers based on the log of the size of the
-		 * relation.  This probably needs to be a good deal more
-		 * sophisticated, but we need something here for now.  Note that the
-		 * upper limit of the min_parallel_relation_size GUC is chosen to
-		 * prevent overflow here.
-		 */
-		parallel_workers = 1;
-		parallel_threshold = Max(min_parallel_relation_size, 1);
-		while (pages >= (BlockNumber) (parallel_threshold * 3))
+		if (heap_pages > 0)
+		{
+			/*
+			 * Select the number of workers based on the log of the size of
+			 * the relation.  This probably needs to be a good deal more
+			 * sophisticated, but we need something here for now.  Note that
+			 * the upper limit of the min_parallel_table_scan_size GUC is
+			 * chosen to prevent overflow here.
+			 */
+			heap_parallel_threshold = Max(min_parallel_table_scan_size, 1);
+			while (heap_pages >= (BlockNumber) (heap_parallel_threshold * 3))
+			{
+				heap_parallel_workers++;
+				heap_parallel_threshold *= 3;
+				if (heap_parallel_threshold > INT_MAX / 3)
+					break;		/* avoid overflow */
+			}
+
+			parallel_workers = heap_parallel_workers;
+		}
+
+		if (index_pages > 0)
 		{
-			parallel_workers++;
-			parallel_threshold *= 3;
-			if (parallel_threshold > INT_MAX / 3)
-				break;			/* avoid overflow */
+			/* same calculation as for heap_pages above */
+			index_parallel_threshold = Max(min_parallel_index_scan_size, 1);
+			while (index_pages >= (BlockNumber) (index_parallel_threshold * 3))
+			{
+				index_parallel_workers++;
+				index_parallel_threshold *= 3;
+				if (index_parallel_threshold > INT_MAX / 3)
+					break;		/* avoid overflow */
+			}
+
+			if (parallel_workers > 0)
+				parallel_workers = Min(parallel_workers, index_parallel_workers);
+			else
+				parallel_workers = index_parallel_workers;
 		}
 	}
 
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index de85eca..0df45b8 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -2776,17 +2776,28 @@ static struct config_int ConfigureNamesInt[] =
 	},
 
 	{
-		{"min_parallel_relation_size", PGC_USERSET, QUERY_TUNING_COST,
-			gettext_noop("Sets the minimum size of relations to be considered for parallel scan."),
+		{"min_parallel_table_scan_size", PGC_USERSET, QUERY_TUNING_COST,
+			gettext_noop("Sets the minimum size of tables to be considered for parallel scan."),
 			NULL,
 			GUC_UNIT_BLOCKS,
 		},
-		&min_parallel_relation_size,
+		&min_parallel_table_scan_size,
 		(8 * 1024 * 1024) / BLCKSZ, 0, INT_MAX / 3,
 		NULL, NULL, NULL
 	},
 
 	{
+		{"min_parallel_index_scan_size", PGC_USERSET, QUERY_TUNING_COST,
+			gettext_noop("Sets the minimum size of indexes to be considered for parallel scan."),
+			NULL,
+			GUC_UNIT_BLOCKS,
+		},
+		&min_parallel_index_scan_size,
+		(512 * 1024) / BLCKSZ, 0, INT_MAX / 3,
+		NULL, NULL, NULL
+	},
+
+	{
 		/* Can't be set in postgresql.conf */
 		{"server_version_num", PGC_INTERNAL, PRESET_OPTIONS,
 			gettext_noop("Shows the server version as an integer."),
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index 661b0fa..157d775 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -300,7 +300,8 @@
 #cpu_operator_cost = 0.0025		# same scale as above
 #parallel_tuple_cost = 0.1		# same scale as above
 #parallel_setup_cost = 1000.0	# same scale as above
-#min_parallel_relation_size = 8MB
+#min_parallel_table_scan_size = 8MB
+#min_parallel_index_scan_size = 512kB
 #effective_cache_size = 4GB
 
 # - Genetic Query Optimizer -
diff --git a/src/include/optimizer/paths.h b/src/include/optimizer/paths.h
index 81a9be7..81e7a42 100644
--- a/src/include/optimizer/paths.h
+++ b/src/include/optimizer/paths.h
@@ -22,7 +22,8 @@
  */
 extern bool enable_geqo;
 extern int	geqo_threshold;
-extern int	min_parallel_relation_size;
+extern int	min_parallel_table_scan_size;
+extern int	min_parallel_index_scan_size;
 
 /* Hook for plugins to get control in set_rel_pathlist() */
 typedef void (*set_rel_pathlist_hook_type) (PlannerInfo *root,
diff --git a/src/test/regress/expected/select_parallel.out b/src/test/regress/expected/select_parallel.out
index 18e21b7..b967b45 100644
--- a/src/test/regress/expected/select_parallel.out
+++ b/src/test/regress/expected/select_parallel.out
@@ -9,7 +9,7 @@ begin isolation level repeatable read;
 -- encourage use of parallel plans
 set parallel_setup_cost=0;
 set parallel_tuple_cost=0;
-set min_parallel_relation_size=0;
+set min_parallel_table_scan_size=0;
 set max_parallel_workers_per_gather=4;
 explain (costs off)
   select count(*) from a_star;
diff --git a/src/test/regress/sql/select_parallel.sql b/src/test/regress/sql/select_parallel.sql
index 8b4090f..e3c32de 100644
--- a/src/test/regress/sql/select_parallel.sql
+++ b/src/test/regress/sql/select_parallel.sql
@@ -12,7 +12,7 @@ begin isolation level repeatable read;
 -- encourage use of parallel plans
 set parallel_setup_cost=0;
 set parallel_tuple_cost=0;
-set min_parallel_relation_size=0;
+set min_parallel_table_scan_size=0;
 set max_parallel_workers_per_gather=4;
 
 explain (costs off)