skip restriction clause if it contains placeholdervar (#6857)

`PlaceHolderVar` is not relevant to be processed inside a restriction
clause. Otherwise, `pull_var_clause_default` would throw error. PG would
create the restriction to physical `Var` that `PlaceHolderVar` points to
anyway, so it is safe to skip this restriction.

DESCRIPTION: Fixes a bug related to WHERE clause list which contains
placeholder.

Fixes https://github.com/citusdata/citus/issues/6758

src/backend/distributed/planner/relation_restriction_equivalence.c

@@ -155,6 +155,7 @@ static bool AllDistributedRelationsInRestrictionContextColocated(
 	RelationRestrictionContext *
 	restrictionContext);
 static bool IsNotSafeRestrictionToRecursivelyPlan(Node *node);
+static bool HasPlaceHolderVar(Node *node);
 static JoinRestrictionContext * FilterJoinRestrictionContext(
 	JoinRestrictionContext *joinRestrictionContext, Relids
 	queryRteIdentities);
@@ -2149,6 +2150,17 @@ GetRestrictInfoListForRelation(RangeTblEntry *rangeTblEntry,
 			continue;
 		}
 
+		/*
+		 * PlaceHolderVar is not relevant to be processed inside a restriction clause.
+		 * Otherwise, pull_var_clause_default would throw error. PG would create
+		 * the restriction to physical Var that PlaceHolderVar points anyway, so it is
+		 * safe to skip this restriction.
+		 */
+		if (FindNodeMatchingCheckFunction((Node *) restrictionClause, HasPlaceHolderVar))
+		{
+			continue;
+		}
+
 		/*
 		 * We're going to add this restriction expression to a subquery
 		 * which consists of only one relation in its jointree. Thus,
@@ -2214,6 +2226,16 @@ IsNotSafeRestrictionToRecursivelyPlan(Node *node)
 }
 
 
+/*
+ * HasPlaceHolderVar returns true if given node contains any PlaceHolderVar.
+ */
+static bool
+HasPlaceHolderVar(Node *node)
+{
+	return IsA(node, PlaceHolderVar);
+}
+
+
 /*
  * FilterRelationRestrictionContext gets a relation restriction context and
  * set of rte identities. It returns the relation restrictions that that appear

src/test/regress/expected/issue_6758.out

@@ -0,0 +1,69 @@
+CREATE SCHEMA issue_6758;
+SET search_path to 'issue_6758';
+CREATE TABLE dist0(id int);
+SELECT create_distributed_table('dist0','id');
+ create_distributed_table
+---------------------------------------------------------------------
+
+(1 row)
+
+CREATE TABLE dist1(id int);
+SELECT create_distributed_table('dist1','id');
+ create_distributed_table
+---------------------------------------------------------------------
+
+(1 row)
+
+-- added to verify we fixed the issue https://github.com/citusdata/citus/issues/6758
+-- generated by Citus query generator tool
+SELECT
+  avg(avgsub.id)
+FROM
+  (
+    SELECT
+      table_0.id
+    FROM
+      (
+        SELECT
+          table_1.id
+        FROM
+          (
+            SELECT
+              table_2.id
+            FROM
+              (
+                SELECT
+                  table_3.id
+                FROM
+                  (
+                    VALUES
+                      (838)
+                  ) AS table_3(id) FULL
+                  JOIN dist0 AS table_4 USING (id)
+                WHERE
+                  table_4.id = 3
+              ) AS table_2
+            WHERE
+              table_2.id = 2
+            ORDER BY
+              id
+            LIMIT
+              77
+          ) AS table_1
+          LEFT JOIN dist0 AS table_5 USING (id)
+        ORDER BY
+          id
+        LIMIT
+          44
+      ) AS table_0 FULL
+      JOIN dist1 AS table_6 USING (id)
+  ) AS avgsub;
+ avg
+---------------------------------------------------------------------
+
+(1 row)
+
+DROP SCHEMA issue_6758 CASCADE;
+NOTICE:  drop cascades to 2 other objects
+DETAIL:  drop cascades to table dist0
+drop cascades to table dist1

src/test/regress/multi_schedule

@@ -95,7 +95,7 @@ test: multi_dropped_column_aliases foreign_key_restriction_enforcement
 test: binary_protocol
 test: alter_table_set_access_method
 test: alter_distributed_table
-test: issue_5248 issue_5099 issue_5763 issue_6543
+test: issue_5248 issue_5099 issue_5763 issue_6543 issue_6758
 test: object_propagation_debug
 test: undistribute_table
 test: run_command_on_all_nodes

src/test/regress/sql/issue_6758.sql

@@ -0,0 +1,55 @@
+CREATE SCHEMA issue_6758;
+SET search_path to 'issue_6758';
+
+CREATE TABLE dist0(id int);
+SELECT create_distributed_table('dist0','id');
+
+CREATE TABLE dist1(id int);
+SELECT create_distributed_table('dist1','id');
+
+-- added to verify we fixed the issue https://github.com/citusdata/citus/issues/6758
+-- generated by Citus query generator tool
+SELECT
+  avg(avgsub.id)
+FROM
+  (
+    SELECT
+      table_0.id
+    FROM
+      (
+        SELECT
+          table_1.id
+        FROM
+          (
+            SELECT
+              table_2.id
+            FROM
+              (
+                SELECT
+                  table_3.id
+                FROM
+                  (
+                    VALUES
+                      (838)
+                  ) AS table_3(id) FULL
+                  JOIN dist0 AS table_4 USING (id)
+                WHERE
+                  table_4.id = 3
+              ) AS table_2
+            WHERE
+              table_2.id = 2
+            ORDER BY
+              id
+            LIMIT
+              77
+          ) AS table_1
+          LEFT JOIN dist0 AS table_5 USING (id)
+        ORDER BY
+          id
+        LIMIT
+          44
+      ) AS table_0 FULL
+      JOIN dist1 AS table_6 USING (id)
+  ) AS avgsub;
+
+DROP SCHEMA issue_6758 CASCADE;