Fix JOINs on varchar columns with subquery pushdown

Fixes #379

Varchar VAR struct is wrapped in RELABELTYPE struct inside PostgreSQL code and
IsPartitionColumnRecursive function considers only VAR types so returning false
for varchar.

This change adds strip_implicit_coercions() call to the columnExpression in
IsPartitionColumnRecursive function so that we get rid of implicit coercions like
RELABELTYPE are stripped to VAR.

src/backend/distributed/planner/multi_logical_optimizer.c

@@ -2907,14 +2907,15 @@ IsPartitionColumnRecursive(Expr *columnExpression, Query *query)
 	List *rangetableList = query->rtable;
 	Index rangeTableEntryIndex = 0;
 	RangeTblEntry *rangeTableEntry = NULL;
+	Expr *strippedColumnExpression = strip_implicit_coercions(columnExpression);
 
-	if (IsA(columnExpression, Var))
+	if (IsA(strippedColumnExpression, Var))
 	{
-		candidateColumn = (Var *) columnExpression;
+		candidateColumn = (Var *) strippedColumnExpression;
 	}
-	else if (IsA(columnExpression, FieldSelect))
+	else if (IsA(strippedColumnExpression, FieldSelect))
 	{
-		FieldSelect *compositeField = (FieldSelect *) columnExpression;
+		FieldSelect *compositeField = (FieldSelect *) strippedColumnExpression;
 		Expr *fieldExpression = compositeField->arg;
 
 		if (IsA(fieldExpression, Var))

src/test/regress/input/multi_subquery.source

@@ -319,6 +319,36 @@ AS foo;
 
 SET client_min_messages TO NOTICE;
 
+-- test subquery join on VARCHAR partition column 
+SELECT * FROM
+	(SELECT 
+		a_inner AS a
+	FROM
+		(SELECT 
+			subquery_pruning_varchar_test_table.a AS a_inner
+		FROM 
+		 	subquery_pruning_varchar_test_table
+		GROUP BY 
+		  	subquery_pruning_varchar_test_table.a
+		HAVING 
+		  	count(subquery_pruning_varchar_test_table.a) < 3) 
+		AS f1,
+
+		(SELECT 
+		 	subquery_pruning_varchar_test_table.a
+		FROM 
+		  	subquery_pruning_varchar_test_table
+		GROUP BY 
+		  	subquery_pruning_varchar_test_table.a
+		HAVING 
+		  	sum(coalesce(subquery_pruning_varchar_test_table.b,0)) > 20.0) 
+		AS f2
+	WHERE 
+		f1.a_inner = f2.a
+	GROUP BY 
+		a_inner)
+AS foo;
+
 DROP TABLE subquery_pruning_varchar_test_table;
 
 ALTER SEQUENCE pg_catalog.pg_dist_shardid_seq RESTART 102026;

src/test/regress/output/multi_subquery.source

@@ -367,5 +367,37 @@ DEBUG:  predicate pruning for shardId 102026
 (0 rows)
 
 SET client_min_messages TO NOTICE;
+-- test subquery join on VARCHAR partition column 
+SELECT * FROM
+	(SELECT 
+		a_inner AS a
+	FROM
+		(SELECT 
+			subquery_pruning_varchar_test_table.a AS a_inner
+		FROM 
+		 	subquery_pruning_varchar_test_table
+		GROUP BY 
+		  	subquery_pruning_varchar_test_table.a
+		HAVING 
+		  	count(subquery_pruning_varchar_test_table.a) < 3) 
+		AS f1,
+		(SELECT 
+		 	subquery_pruning_varchar_test_table.a
+		FROM 
+		  	subquery_pruning_varchar_test_table
+		GROUP BY 
+		  	subquery_pruning_varchar_test_table.a
+		HAVING 
+		  	sum(coalesce(subquery_pruning_varchar_test_table.b,0)) > 20.0) 
+		AS f2
+	WHERE 
+		f1.a_inner = f2.a
+	GROUP BY 
+		a_inner)
+AS foo;
+ a 
+---
+(0 rows)
+
 DROP TABLE subquery_pruning_varchar_test_table;
 ALTER SEQUENCE pg_catalog.pg_dist_shardid_seq RESTART 102026;