Do not translate dummy SpecialJoinInfos for child joins

This teaches build_child_join_sjinfo() to create the dummy
SpecialJoinInfos (those created for inner joins) directly for a given
child join, skipping the unnecessary overhead of translating the
parent joinrel's SpecialJoinInfo.

To that end, this commit moves the code to initialize the dummy
SpecialJoinInfos to a new function named init_dummy_sjinfo() and
changes the few existing sites that have this code and
build_child_join_sjinfo() to call this new function.

Author: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Reviewed-by: Richard Guo <guofenglinux@gmail.com>
Reviewed-by: Amit Langote <amitlangote09@gmail.com>
Reviewed-by: Andrey Lepikhov <a.lepikhov@postgrespro.ru>
Reviewed-by: Tomas Vondra <tomas.vondra@enterprisedb.com>
Discussion: https://postgr.es/m/CAExHW5tHqEf3ASVqvFFcghYGPfpy7o3xnvhHwBGbJFMRH8KjNw@mail.gmail.com

src/backend/optimizer/path/costsize.c

@@ -5050,23 +5050,7 @@ compute_semi_anti_join_factors(PlannerInfo *root,
 	/*
 	 * Also get the normal inner-join selectivity of the join clauses.
 	 */
-	norm_sjinfo.type = T_SpecialJoinInfo;
+	init_dummy_sjinfo(&norm_sjinfo, outerrel->relids, innerrel->relids);
-	norm_sjinfo.min_lefthand = outerrel->relids;
-	norm_sjinfo.min_righthand = innerrel->relids;
-	norm_sjinfo.syn_lefthand = outerrel->relids;
-	norm_sjinfo.syn_righthand = innerrel->relids;
-	norm_sjinfo.jointype = JOIN_INNER;
-	norm_sjinfo.ojrelid = 0;
-	norm_sjinfo.commute_above_l = NULL;
-	norm_sjinfo.commute_above_r = NULL;
-	norm_sjinfo.commute_below_l = NULL;
-	norm_sjinfo.commute_below_r = NULL;
-	/* we don't bother trying to make the remaining fields valid */
-	norm_sjinfo.lhs_strict = false;
-	norm_sjinfo.semi_can_btree = false;
-	norm_sjinfo.semi_can_hash = false;
-	norm_sjinfo.semi_operators = NIL;
-	norm_sjinfo.semi_rhs_exprs = NIL;
 
 	nselec = clauselist_selectivity(root,
 									joinquals,
@@ -5219,23 +5203,8 @@ approx_tuple_count(PlannerInfo *root, JoinPath *path, List *quals)
 	/*
 	 * Make up a SpecialJoinInfo for JOIN_INNER semantics.
 	 */
-	sjinfo.type = T_SpecialJoinInfo;
+	init_dummy_sjinfo(&sjinfo, path->outerjoinpath->parent->relids,
-	sjinfo.min_lefthand = path->outerjoinpath->parent->relids;
+					  path->innerjoinpath->parent->relids);
-	sjinfo.min_righthand = path->innerjoinpath->parent->relids;
-	sjinfo.syn_lefthand = path->outerjoinpath->parent->relids;
-	sjinfo.syn_righthand = path->innerjoinpath->parent->relids;
-	sjinfo.jointype = JOIN_INNER;
-	sjinfo.ojrelid = 0;
-	sjinfo.commute_above_l = NULL;
-	sjinfo.commute_above_r = NULL;
-	sjinfo.commute_below_l = NULL;
-	sjinfo.commute_below_r = NULL;
-	/* we don't bother trying to make the remaining fields valid */
-	sjinfo.lhs_strict = false;
-	sjinfo.semi_can_btree = false;
-	sjinfo.semi_can_hash = false;
-	sjinfo.semi_operators = NIL;
-	sjinfo.semi_rhs_exprs = NIL;
 
 	/* Get the approximate selectivity */
 	foreach(l, quals)

src/backend/optimizer/path/joinrels.c

@@ -654,6 +654,38 @@ join_is_legal(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2,
 	return true;
 }
 
+/*
+ * init_dummy_sjinfo
+ *    Populate the given SpecialJoinInfo for a plain inner join between rel1
+ *    and rel2
+ *
+ * Normally, an inner join does not have a SpecialJoinInfo node associated with
+ * it. But some functions involved in join planning require one containing at
+ * least the information of which relations are being joined.  So we initialize
+ * that information here.
+ */
+void
+init_dummy_sjinfo(SpecialJoinInfo *sjinfo, Relids left_relids,
+				  Relids right_relids)
+{
+	sjinfo->type = T_SpecialJoinInfo;
+	sjinfo->min_lefthand = left_relids;
+	sjinfo->min_righthand = right_relids;
+	sjinfo->syn_lefthand = left_relids;
+	sjinfo->syn_righthand = right_relids;
+	sjinfo->jointype = JOIN_INNER;
+	sjinfo->ojrelid = 0;
+	sjinfo->commute_above_l = NULL;
+	sjinfo->commute_above_r = NULL;
+	sjinfo->commute_below_l = NULL;
+	sjinfo->commute_below_r = NULL;
+	/* we don't bother trying to make the remaining fields valid */
+	sjinfo->lhs_strict = false;
+	sjinfo->semi_can_btree = false;
+	sjinfo->semi_can_hash = false;
+	sjinfo->semi_operators = NIL;
+	sjinfo->semi_rhs_exprs = NIL;
+}
 
 /*
  * make_join_rel
@@ -717,23 +749,7 @@ make_join_rel(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2)
 	if (sjinfo == NULL)
 	{
 		sjinfo = &sjinfo_data;
-		sjinfo->type = T_SpecialJoinInfo;
+		init_dummy_sjinfo(sjinfo, rel1->relids, rel2->relids);
-		sjinfo->min_lefthand = rel1->relids;
-		sjinfo->min_righthand = rel2->relids;
-		sjinfo->syn_lefthand = rel1->relids;
-		sjinfo->syn_righthand = rel2->relids;
-		sjinfo->jointype = JOIN_INNER;
-		sjinfo->ojrelid = 0;
-		sjinfo->commute_above_l = NULL;
-		sjinfo->commute_above_r = NULL;
-		sjinfo->commute_below_l = NULL;
-		sjinfo->commute_below_r = NULL;
-		/* we don't bother trying to make the remaining fields valid */
-		sjinfo->lhs_strict = false;
-		sjinfo->semi_can_btree = false;
-		sjinfo->semi_can_hash = false;
-		sjinfo->semi_operators = NIL;
-		sjinfo->semi_rhs_exprs = NIL;
 	}
 
 	/*
@@ -1682,6 +1698,14 @@ build_child_join_sjinfo(PlannerInfo *root, SpecialJoinInfo *parent_sjinfo,
 	AppendRelInfo **right_appinfos;
 	int			right_nappinfos;
 
+	/* Dummy SpecialJoinInfos can be created without any translation. */
+	if (parent_sjinfo->jointype == JOIN_INNER)
+	{
+		Assert(parent_sjinfo->ojrelid == 0);
+		init_dummy_sjinfo(sjinfo, left_relids, right_relids);
+		return sjinfo;
+	}
+
 	memcpy(sjinfo, parent_sjinfo, sizeof(SpecialJoinInfo));
 	left_appinfos = find_appinfos_by_relids(root, left_relids,
 											&left_nappinfos);

src/include/optimizer/paths.h

@@ -112,6 +112,8 @@ extern bool have_join_order_restriction(PlannerInfo *root,
 extern bool have_dangerous_phv(PlannerInfo *root,
 							   Relids outer_relids, Relids inner_params);
 extern void mark_dummy_rel(RelOptInfo *rel);
+extern void init_dummy_sjinfo(SpecialJoinInfo *sjinfo, Relids left_relids,
+							  Relids right_relids);
 
 /*
  * equivclass.c