Rearrange make_partitionedrel_pruneinfo to avoid work when we can't prune.

Postpone most of the effort of constructing PartitionedRelPruneInfos
until after we have found out whether run-time pruning is needed at all.
This costs very little duplicated effort (basically just an extra
find_base_rel() call per partition) and saves quite a bit when we
can't do run-time pruning.

Also, merge the first loop (for building relid_subpart_map) into
the second loop, since we don't need the map to be valid during
that loop.

Amit Langote

Discussion: https://postgr.es/m/9d7c5112-cb99-6a47-d3be-cf1ee6862a1d@lab.ntt.co.jp

src/backend/partitioning/partprune.c

@@ -326,46 +326,43 @@ make_partitionedrel_pruneinfo(PlannerInfo *root, RelOptInfo *parentrel,
 	int			i;
 
 	/*
-	 * Construct a temporary array to map from planner relids to index of the
-	 * partitioned_rel.  For convenience, we use 1-based indexes here, so that
-	 * zero can represent an un-filled array entry.
+	 * Examine each partitioned rel, constructing a temporary array to map
+	 * from planner relids to index of the partitioned rel, and building a
+	 * PartitionedRelPruneInfo for each partitioned rel.
+	 *
+	 * In this phase we discover whether runtime pruning is needed at all; if
+	 * not, we can avoid doing further work.
 	 */
 	relid_subpart_map = palloc0(sizeof(int) * root->simple_rel_array_size);
 
-	/*
-	 * relid_subpart_map maps relid of a non-leaf partition to the index in
-	 * 'partitioned_rels' of that rel (which will also be the index in the
-	 * returned PartitionedRelPruneInfo list of the info for that partition).
-	 */
 	i = 1;
 	foreach(lc, partitioned_rels)
-	{
-		Index		rti = lfirst_int(lc);
-
-		Assert(rti < root->simple_rel_array_size);
-		/* No duplicates please */
-		Assert(relid_subpart_map[rti] == 0);
-
-		relid_subpart_map[rti] = i++;
-	}
-
-	/* We now build a PartitionedRelPruneInfo for each partitioned rel */
-	foreach(lc, partitioned_rels)
 	{
 		Index		rti = lfirst_int(lc);
 		RelOptInfo *subpart = find_base_rel(root, rti);
 		PartitionedRelPruneInfo *pinfo;
-		Bitmapset  *present_parts;
-		int			nparts = subpart->nparts;
 		int			partnatts = subpart->part_scheme->partnatts;
-		int		   *subplan_map;
-		int		   *subpart_map;
-		Oid		   *relid_map;
 		List	   *partprunequal;
 		List	   *pruning_steps;
 		bool		contradictory;
 
 		/*
+		 * Fill the mapping array.
+		 *
+		 * relid_subpart_map maps relid of a non-leaf partition to the index
+		 * in 'partitioned_rels' of that rel (which will also be the index in
+		 * the returned PartitionedRelPruneInfo list of the info for that
+		 * partition).  We use 1-based indexes here, so that zero can
+		 * represent an un-filled array entry.
+		 */
+		Assert(rti < root->simple_rel_array_size);
+		/* No duplicates please */
+		Assert(relid_subpart_map[rti] == 0);
+		relid_subpart_map[rti] = i++;
+
+		/*
+		 * Translate pruning qual, if necessary, for this partition.
+		 *
 		 * The first item in the list is the target partitioned relation.
 		 */
 		if (!targetpart)
@@ -411,6 +408,7 @@ make_partitionedrel_pruneinfo(PlannerInfo *root, RelOptInfo *parentrel,
 												  targetpart->relids);
 		}
 
+		/* Convert pruning qual to pruning steps. */
 		pruning_steps = gen_partprune_steps(subpart, partprunequal,
 											&contradictory);
 
@@ -428,6 +426,47 @@ make_partitionedrel_pruneinfo(PlannerInfo *root, RelOptInfo *parentrel,
 			return NIL;
 		}
 
+		/* Begin constructing the PartitionedRelPruneInfo for this rel */
+		pinfo = makeNode(PartitionedRelPruneInfo);
+		pinfo->rtindex = rti;
+		pinfo->pruning_steps = pruning_steps;
+		/* Remaining fields will be filled in the next loop */
+
+		pinfolist = lappend(pinfolist, pinfo);
+
+		/*
+		 * Determine which pruning types should be enabled at this level. This
+		 * also records paramids relevant to pruning steps in 'pinfo'.
+		 */
+		doruntimeprune |= analyze_partkey_exprs(pinfo, pruning_steps,
+												partnatts);
+	}
+
+	if (!doruntimeprune)
+	{
+		/* No run-time pruning required. */
+		pfree(relid_subpart_map);
+		return NIL;
+	}
+
+	/*
+	 * Run-time pruning will be required, so initialize other information.
+	 * That includes two maps -- one needed to convert partition indexes of
+	 * leaf partitions to the indexes of their subplans in the subplan list,
+	 * another needed to convert partition indexes of sub-partitioned
+	 * partitions to the indexes of their PartitionedRelPruneInfo in the
+	 * PartitionedRelPruneInfo list.
+	 */
+	foreach(lc, pinfolist)
+	{
+		PartitionedRelPruneInfo *pinfo = lfirst(lc);
+		RelOptInfo *subpart = find_base_rel(root, pinfo->rtindex);
+		Bitmapset  *present_parts;
+		int			nparts = subpart->nparts;
+		int		   *subplan_map;
+		int		   *subpart_map;
+		Oid		   *relid_map;
+
 		/*
 		 * Construct the subplan and subpart maps for this partitioning level.
 		 * Here we convert to zero-based indexes, with -1 for empty entries.
@@ -459,30 +498,16 @@ make_partitionedrel_pruneinfo(PlannerInfo *root, RelOptInfo *parentrel,
 				present_parts = bms_add_member(present_parts, i);
 		}
 
-		pinfo = makeNode(PartitionedRelPruneInfo);
-		pinfo->rtindex = rti;
-		pinfo->pruning_steps = pruning_steps;
+		/* Record the maps and other information. */
 		pinfo->present_parts = present_parts;
 		pinfo->nparts = nparts;
 		pinfo->subplan_map = subplan_map;
 		pinfo->subpart_map = subpart_map;
 		pinfo->relid_map = relid_map;
-
-		/* Determine which pruning types should be enabled at this level */
-		doruntimeprune |= analyze_partkey_exprs(pinfo, pruning_steps,
-												partnatts);
-
-		pinfolist = lappend(pinfolist, pinfo);
 	}
 
 	pfree(relid_subpart_map);
 
-	if (!doruntimeprune)
-	{
-		/* No run-time pruning required. */
-		return NIL;
-	}
-
 	*matchedsubplans = subplansfound;
 
 	return pinfolist;
@@ -2907,6 +2932,9 @@ pull_exec_paramids_walker(Node *node, Bitmapset **context)
  *
  * Returns true if any executor partition pruning should be attempted at this
  * level.  Also fills fields of *pinfo to record how to process each step.
+ *
+ * Note: when this is called, not much of *pinfo is valid; but that's OK
+ * since we only use it as an output area.
  */
 static bool
 analyze_partkey_exprs(PartitionedRelPruneInfo *pinfo, List *steps,