Fix crash on create statistics with non-RangeVar type (#8213)

This crash has been there for a while but wasn't tested before pg18.

PG18 added this test:
CREATE STATISTICS tst ON a FROM (VALUES (x)) AS foo;

which tries to create statistics on a derived-on-the-fly table (which is
not allowed) However Citus assumes we always have a valid table when
intercepting CREATE STATISTICS command to check for Citus tables
Added a check to return early if needed.

pg18 commit: https://github.com/postgres/postgres/commit/3eea4dc2c

Fixes #8212

src/backend/distributed/commands/statistics.c

@@ -69,7 +69,15 @@ PreprocessCreateStatisticsStmt(Node *node, const char *queryString,
 {
 	CreateStatsStmt *stmt = castNode(CreateStatsStmt, node);
 
-	RangeVar *relation = (RangeVar *) linitial(stmt->relations);
+	Node *relationNode = (Node *) linitial(stmt->relations);
+
+	if (!IsA(relationNode, RangeVar))
+	{
+		return NIL;
+	}
+
+	RangeVar *relation = (RangeVar *) relationNode;
+
 	Oid relationId = RangeVarGetRelid(relation, ShareUpdateExclusiveLock, false);
 
 	if (!IsCitusTable(relationId) || !ShouldPropagate())

src/test/regress/expected/pg18.out

@@ -4,11 +4,17 @@
 SHOW server_version \gset
 SELECT substring(:'server_version', '\d+')::int >= 18 AS server_version_ge_18
 \gset
+-- test invalid statistics
+-- behavior is same among PG versions, error message differs
+-- relevant PG18 commit: 3eea4dc2c7
+CREATE STATISTICS tst ON a FROM (VALUES (x)) AS foo;
+ERROR:  cannot create statistics on the specified relation
+DETAIL:  CREATE STATISTICS only supports tables, foreign tables and materialized views.
 \if :server_version_ge_18
 \else
 \q
 \endif
--- PG17-specific tests go here.
+-- PG18-specific tests go here.
 --
 -- Purpose: Verify PG18 behavior that NOT NULL constraints are materialized
 --          as pg_constraint rows with contype = 'n' on both coordinator and

src/test/regress/expected/pg18_0.out

@@ -4,6 +4,11 @@
 SHOW server_version \gset
 SELECT substring(:'server_version', '\d+')::int >= 18 AS server_version_ge_18
 \gset
+-- test invalid statistics
+-- behavior is same among PG versions, error message differs
+-- relevant PG18 commit: 3eea4dc2c7
+CREATE STATISTICS tst ON a FROM (VALUES (x)) AS foo;
+ERROR:  only a single relation is allowed in CREATE STATISTICS
 \if :server_version_ge_18
 \else
 \q

src/test/regress/sql/pg18.sql

@@ -5,12 +5,17 @@ SHOW server_version \gset
 SELECT substring(:'server_version', '\d+')::int >= 18 AS server_version_ge_18
 \gset
 
+-- test invalid statistics
+-- behavior is same among PG versions, error message differs
+-- relevant PG18 commit: 3eea4dc2c7
+CREATE STATISTICS tst ON a FROM (VALUES (x)) AS foo;
+
 \if :server_version_ge_18
 \else
 \q
 \endif
 
--- PG17-specific tests go here.
+-- PG18-specific tests go here.
 --
 
 -- Purpose: Verify PG18 behavior that NOT NULL constraints are materialized