Fixes ALTER STATISTICS IF EXISTS bug (#5435)

* Fix ALTER STATISTICS IF EXISTS bug

src/backend/distributed/commands/statistics.c

@@ -348,7 +348,18 @@ PreprocessAlterStatisticsStmt(Node *node, const char *queryString,
 {
 	AlterStatsStmt *stmt = castNode(AlterStatsStmt, node);
 
-	Oid statsOid = get_statistics_object_oid(stmt->defnames, false);
+	Oid statsOid = get_statistics_object_oid(stmt->defnames, stmt->missing_ok);
+
+	if (!OidIsValid(statsOid))
+	{
+		/*
+		 * If statsOid is invalid, here we can assume that the query includes
+		 * IF EXISTS clause, since get_statistics_object_oid would error out otherwise.
+		 * So here we can safely return NIL here without checking stmt->missing_ok.
+		 */
+		return NIL;
+	}
+
 	Oid relationId = GetRelIdByStatsOid(statsOid);
 
 	if (!IsCitusTable(relationId) || !ShouldPropagate())

src/test/regress/expected/pg13_propagate_statistics.out

@@ -108,5 +108,16 @@ ORDER BY stxstattarget, stxrelid::regclass ASC;
 (64 rows)
 
 \c - - - :master_port
+-- the first one should log a notice that says statistics object does not exist
+ALTER STATISTICS IF EXISTS stats_that_doesnt_exists SET STATISTICS 0;
+NOTICE:  statistics object "stats_that_doesnt_exists" does not exist, skipping
+-- these three should error out as ALTER STATISTICS syntax doesn't support these with IF EXISTS clause
+-- if output of any of these three changes, we should support them and update the test output here
+ALTER STATISTICS IF EXISTS stats_that_doesnt_exists RENAME TO this_should_error_out;
+ERROR:  syntax error at or near "RENAME"
+ALTER STATISTICS IF EXISTS stats_that_doesnt_exists OWNER TO CURRENT_USER;
+ERROR:  syntax error at or near "OWNER"
+ALTER STATISTICS IF EXISTS stats_that_doesnt_exists SET SCHEMA "statistics'Test";
+ERROR:  syntax error at or near "SCHEMA"
 SET client_min_messages TO WARNING;
 DROP SCHEMA "statistics'TestTarget" CASCADE;

src/test/regress/sql/pg13_propagate_statistics.sql

@@ -41,5 +41,12 @@ WHERE stxnamespace IN (
 ORDER BY stxstattarget, stxrelid::regclass ASC;
 
 \c - - - :master_port
+-- the first one should log a notice that says statistics object does not exist
+ALTER STATISTICS IF EXISTS stats_that_doesnt_exists SET STATISTICS 0;
+-- these three should error out as ALTER STATISTICS syntax doesn't support these with IF EXISTS clause
+-- if output of any of these three changes, we should support them and update the test output here
+ALTER STATISTICS IF EXISTS stats_that_doesnt_exists RENAME TO this_should_error_out;
+ALTER STATISTICS IF EXISTS stats_that_doesnt_exists OWNER TO CURRENT_USER;
+ALTER STATISTICS IF EXISTS stats_that_doesnt_exists SET SCHEMA "statistics'Test";
 SET client_min_messages TO WARNING;
 DROP SCHEMA "statistics'TestTarget" CASCADE;