Sanitize IF NOT EXISTS in EXPLAIN for CTAS and matviews

IF NOT EXISTS was ignored when specified in an EXPLAIN query for CREATE
MATERIALIZED VIEW or CREATE TABLE AS.  Hence, if this clause was
specified, the caller would get a failure if the relation already
exists instead of a success with a NOTICE message.

This commit makes the behavior of IF NOT EXISTS in EXPLAIN consistent
with the non-EXPLAIN'd DDL queries, preventing a failure with IF NOT
EXISTS if the relation to-be-created already exists.  The skip is done
before the SELECT query used for the relation is planned or executed,
and a "dummy" plan is generated instead depending on the format used by
EXPLAIN.

Author: Bharath Rupireddy
Reviewed-by: Zhijie Hou, Michael Paquier
Discussion: https://postgr.es/m/CALj2ACVa3oJ9O_wcGd+FtHWZds04dEKcakxphGz5POVgD4wC7Q@mail.gmail.com

src/backend/commands/createas.c

@@ -239,21 +239,9 @@ ExecCreateTableAs(ParseState *pstate, CreateTableAsStmt *stmt,
 	PlannedStmt *plan;
 	QueryDesc  *queryDesc;
 
-	if (stmt->if_not_exists)
-	{
-		Oid			nspid;
-
-		nspid = RangeVarGetCreationNamespace(stmt->into->rel);
-
-		if (get_relname_relid(stmt->into->rel->relname, nspid))
-		{
-			ereport(NOTICE,
-					(errcode(ERRCODE_DUPLICATE_TABLE),
-					 errmsg("relation \"%s\" already exists, skipping",
-							stmt->into->rel->relname)));
-			return InvalidObjectAddress;
-		}
-	}
+	/* Check if the relation exists or not */
+	if (CreateTableAsRelExists(stmt))
+		return InvalidObjectAddress;
 
 	/*
 	 * Create the tuple receiver object and insert info it will need
@@ -400,6 +388,41 @@ GetIntoRelEFlags(IntoClause *intoClause)
 	return flags;
 }
 
+/*
+ * CreateTableAsRelExists --- check existence of relation for CreateTableAsStmt
+ *
+ * Utility wrapper checking if the relation pending for creation in this
+ * CreateTableAsStmt query already exists or not.  Returns true if the
+ * relation exists, otherwise false.
+ */
+bool
+CreateTableAsRelExists(CreateTableAsStmt *ctas)
+{
+	Oid			nspid;
+	IntoClause *into = ctas->into;
+
+	nspid = RangeVarGetCreationNamespace(into->rel);
+
+	if (get_relname_relid(into->rel->relname, nspid))
+	{
+		if (!ctas->if_not_exists)
+			ereport(ERROR,
+					(errcode(ERRCODE_DUPLICATE_TABLE),
+					 errmsg("relation \"%s\" already exists",
+							into->rel->relname)));
+
+		/* The relation exists and IF NOT EXISTS has been specified */
+		ereport(NOTICE,
+				(errcode(ERRCODE_DUPLICATE_TABLE),
+				 errmsg("relation \"%s\" already exists, skipping",
+						into->rel->relname)));
+		return true;
+	}
+
+	/* Relation does not exist, it can be created */
+	return false;
+}
+
 /*
  * CreateIntoRelDestReceiver -- create a suitable DestReceiver object
  *

src/backend/commands/explain.c

@@ -435,6 +435,22 @@ ExplainOneUtility(Node *utilityStmt, IntoClause *into, ExplainState *es,
 		CreateTableAsStmt *ctas = (CreateTableAsStmt *) utilityStmt;
 		List	   *rewritten;
 
+		/*
+		 * Check if the relation exists or not.  This is done at this stage to
+		 * avoid query planning or execution.
+		 */
+		if (CreateTableAsRelExists(ctas))
+		{
+			if (ctas->objtype == OBJECT_TABLE)
+				ExplainDummyGroup("CREATE TABLE AS", NULL, es);
+			else if (ctas->objtype == OBJECT_MATVIEW)
+				ExplainDummyGroup("CREATE MATERIALIZED VIEW", NULL, es);
+			else
+				elog(ERROR,	"unexpected object type: %d",
+					 (int) ctas->objtype);
+			return;
+		}
+
 		rewritten = QueryRewrite(castNode(Query, copyObject(ctas->query)));
 		Assert(list_length(rewritten) == 1);
 		ExplainOneQuery(linitial_node(Query, rewritten),

src/include/commands/createas.h

@@ -29,4 +29,6 @@ extern int	GetIntoRelEFlags(IntoClause *intoClause);
 
 extern DestReceiver *CreateIntoRelDestReceiver(IntoClause *intoClause);
 
+extern bool CreateTableAsRelExists(CreateTableAsStmt *ctas);
+
 #endif							/* CREATEAS_H */

src/test/regress/expected/matview.out

@@ -630,3 +630,41 @@ drop cascades to materialized view matview_schema.mv_withdata2
 drop cascades to materialized view matview_schema.mv_nodata1
 drop cascades to materialized view matview_schema.mv_nodata2
 DROP USER regress_matview_user;
+-- CREATE MATERIALIZED VIEW ... IF NOT EXISTS
+CREATE MATERIALIZED VIEW matview_ine_tab AS SELECT 1;
+CREATE MATERIALIZED VIEW matview_ine_tab AS SELECT 1 / 0; -- error
+ERROR:  relation "matview_ine_tab" already exists
+CREATE MATERIALIZED VIEW IF NOT EXISTS matview_ine_tab AS
+  SELECT 1 / 0; -- ok
+NOTICE:  relation "matview_ine_tab" already exists, skipping
+CREATE MATERIALIZED VIEW matview_ine_tab AS
+  SELECT 1 / 0 WITH NO DATA; -- error
+ERROR:  relation "matview_ine_tab" already exists
+CREATE MATERIALIZED VIEW IF NOT EXISTS matview_ine_tab AS
+  SELECT 1 / 0 WITH NO DATA; -- ok
+NOTICE:  relation "matview_ine_tab" already exists, skipping
+EXPLAIN (ANALYZE, COSTS OFF, SUMMARY OFF, TIMING OFF)
+  CREATE MATERIALIZED VIEW matview_ine_tab AS
+    SELECT 1 / 0; -- error
+ERROR:  relation "matview_ine_tab" already exists
+EXPLAIN (ANALYZE, COSTS OFF, SUMMARY OFF, TIMING OFF)
+  CREATE MATERIALIZED VIEW IF NOT EXISTS matview_ine_tab AS
+    SELECT 1 / 0; -- ok
+NOTICE:  relation "matview_ine_tab" already exists, skipping
+ QUERY PLAN 
+------------
+(0 rows)
+
+EXPLAIN (ANALYZE, COSTS OFF, SUMMARY OFF, TIMING OFF)
+  CREATE MATERIALIZED VIEW matview_ine_tab AS
+    SELECT 1 / 0 WITH NO DATA; -- error
+ERROR:  relation "matview_ine_tab" already exists
+EXPLAIN (ANALYZE, COSTS OFF, SUMMARY OFF, TIMING OFF)
+  CREATE MATERIALIZED VIEW IF NOT EXISTS matview_ine_tab AS
+    SELECT 1 / 0 WITH NO DATA; -- ok
+NOTICE:  relation "matview_ine_tab" already exists, skipping
+ QUERY PLAN 
+------------
+(0 rows)
+
+DROP MATERIALIZED VIEW matview_ine_tab;

src/test/regress/expected/select_into.out

@@ -178,3 +178,45 @@ INSERT INTO b SELECT 1 INTO f;
 ERROR:  SELECT ... INTO is not allowed here
 LINE 1: INSERT INTO b SELECT 1 INTO f;
                                     ^
+-- Test CREATE TABLE AS ... IF NOT EXISTS
+CREATE TABLE ctas_ine_tbl AS SELECT 1;
+CREATE TABLE ctas_ine_tbl AS SELECT 1 / 0; -- error
+ERROR:  relation "ctas_ine_tbl" already exists
+CREATE TABLE IF NOT EXISTS ctas_ine_tbl AS SELECT 1 / 0; -- ok
+NOTICE:  relation "ctas_ine_tbl" already exists, skipping
+CREATE TABLE ctas_ine_tbl AS SELECT 1 / 0 WITH NO DATA; -- error
+ERROR:  relation "ctas_ine_tbl" already exists
+CREATE TABLE IF NOT EXISTS ctas_ine_tbl AS SELECT 1 / 0 WITH NO DATA; -- ok
+NOTICE:  relation "ctas_ine_tbl" already exists, skipping
+EXPLAIN (ANALYZE, COSTS OFF, SUMMARY OFF, TIMING OFF)
+  CREATE TABLE ctas_ine_tbl AS SELECT 1 / 0; -- error
+ERROR:  relation "ctas_ine_tbl" already exists
+EXPLAIN (ANALYZE, COSTS OFF, SUMMARY OFF, TIMING OFF)
+  CREATE TABLE IF NOT EXISTS ctas_ine_tbl AS SELECT 1 / 0; -- ok
+NOTICE:  relation "ctas_ine_tbl" already exists, skipping
+ QUERY PLAN 
+------------
+(0 rows)
+
+EXPLAIN (ANALYZE, COSTS OFF, SUMMARY OFF, TIMING OFF)
+  CREATE TABLE ctas_ine_tbl AS SELECT 1 / 0 WITH NO DATA; -- error
+ERROR:  relation "ctas_ine_tbl" already exists
+EXPLAIN (ANALYZE, COSTS OFF, SUMMARY OFF, TIMING OFF)
+  CREATE TABLE IF NOT EXISTS ctas_ine_tbl AS SELECT 1 / 0 WITH NO DATA; -- ok
+NOTICE:  relation "ctas_ine_tbl" already exists, skipping
+ QUERY PLAN 
+------------
+(0 rows)
+
+PREPARE ctas_ine_query AS SELECT 1 / 0;
+EXPLAIN (ANALYZE, COSTS OFF, SUMMARY OFF, TIMING OFF)
+  CREATE TABLE ctas_ine_tbl AS EXECUTE ctas_ine_query; -- error
+ERROR:  relation "ctas_ine_tbl" already exists
+EXPLAIN (ANALYZE, COSTS OFF, SUMMARY OFF, TIMING OFF)
+  CREATE TABLE IF NOT EXISTS ctas_ine_tbl AS EXECUTE ctas_ine_query; -- ok
+NOTICE:  relation "ctas_ine_tbl" already exists, skipping
+ QUERY PLAN 
+------------
+(0 rows)
+
+DROP TABLE ctas_ine_tbl;

src/test/regress/sql/matview.sql

@@ -264,3 +264,26 @@ ALTER DEFAULT PRIVILEGES FOR ROLE regress_matview_user
 
 DROP SCHEMA matview_schema CASCADE;
 DROP USER regress_matview_user;
+
+-- CREATE MATERIALIZED VIEW ... IF NOT EXISTS
+CREATE MATERIALIZED VIEW matview_ine_tab AS SELECT 1;
+CREATE MATERIALIZED VIEW matview_ine_tab AS SELECT 1 / 0; -- error
+CREATE MATERIALIZED VIEW IF NOT EXISTS matview_ine_tab AS
+  SELECT 1 / 0; -- ok
+CREATE MATERIALIZED VIEW matview_ine_tab AS
+  SELECT 1 / 0 WITH NO DATA; -- error
+CREATE MATERIALIZED VIEW IF NOT EXISTS matview_ine_tab AS
+  SELECT 1 / 0 WITH NO DATA; -- ok
+EXPLAIN (ANALYZE, COSTS OFF, SUMMARY OFF, TIMING OFF)
+  CREATE MATERIALIZED VIEW matview_ine_tab AS
+    SELECT 1 / 0; -- error
+EXPLAIN (ANALYZE, COSTS OFF, SUMMARY OFF, TIMING OFF)
+  CREATE MATERIALIZED VIEW IF NOT EXISTS matview_ine_tab AS
+    SELECT 1 / 0; -- ok
+EXPLAIN (ANALYZE, COSTS OFF, SUMMARY OFF, TIMING OFF)
+  CREATE MATERIALIZED VIEW matview_ine_tab AS
+    SELECT 1 / 0 WITH NO DATA; -- error
+EXPLAIN (ANALYZE, COSTS OFF, SUMMARY OFF, TIMING OFF)
+  CREATE MATERIALIZED VIEW IF NOT EXISTS matview_ine_tab AS
+    SELECT 1 / 0 WITH NO DATA; -- ok
+DROP MATERIALIZED VIEW matview_ine_tab;

src/test/regress/sql/select_into.sql

@@ -115,3 +115,24 @@ COPY (SELECT 1 INTO frak UNION SELECT 2) TO 'blob';
 SELECT * FROM (SELECT 1 INTO f) bar;
 CREATE VIEW foo AS SELECT 1 INTO b;
 INSERT INTO b SELECT 1 INTO f;
+
+-- Test CREATE TABLE AS ... IF NOT EXISTS
+CREATE TABLE ctas_ine_tbl AS SELECT 1;
+CREATE TABLE ctas_ine_tbl AS SELECT 1 / 0; -- error
+CREATE TABLE IF NOT EXISTS ctas_ine_tbl AS SELECT 1 / 0; -- ok
+CREATE TABLE ctas_ine_tbl AS SELECT 1 / 0 WITH NO DATA; -- error
+CREATE TABLE IF NOT EXISTS ctas_ine_tbl AS SELECT 1 / 0 WITH NO DATA; -- ok
+EXPLAIN (ANALYZE, COSTS OFF, SUMMARY OFF, TIMING OFF)
+  CREATE TABLE ctas_ine_tbl AS SELECT 1 / 0; -- error
+EXPLAIN (ANALYZE, COSTS OFF, SUMMARY OFF, TIMING OFF)
+  CREATE TABLE IF NOT EXISTS ctas_ine_tbl AS SELECT 1 / 0; -- ok
+EXPLAIN (ANALYZE, COSTS OFF, SUMMARY OFF, TIMING OFF)
+  CREATE TABLE ctas_ine_tbl AS SELECT 1 / 0 WITH NO DATA; -- error
+EXPLAIN (ANALYZE, COSTS OFF, SUMMARY OFF, TIMING OFF)
+  CREATE TABLE IF NOT EXISTS ctas_ine_tbl AS SELECT 1 / 0 WITH NO DATA; -- ok
+PREPARE ctas_ine_query AS SELECT 1 / 0;
+EXPLAIN (ANALYZE, COSTS OFF, SUMMARY OFF, TIMING OFF)
+  CREATE TABLE ctas_ine_tbl AS EXECUTE ctas_ine_query; -- error
+EXPLAIN (ANALYZE, COSTS OFF, SUMMARY OFF, TIMING OFF)
+  CREATE TABLE IF NOT EXISTS ctas_ine_tbl AS EXECUTE ctas_ine_query; -- ok
+DROP TABLE ctas_ine_tbl;