Address review feedback: fix variable registration for deferred label check

When the deferred label validity check (DML predecessor + non-existent
label) found an invalid label, the code skipped transform_match_pattern()
entirely, which meant MATCH-introduced variables were never registered
in the namespace. This would cause errors if a later clause referenced
those variables (e.g., RETURN p).

Fix: mirror the early-check strategy by injecting a paradoxical WHERE
(true = false) and always calling transform_match_pattern(). Variables
get registered normally; zero rows are returned via the impossible qual.

Also add ORDER BY to multi-row regression tests for deterministic output,
and add a test case for DML predecessor + non-existent label + returning
a MATCH-introduced variable.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: gregfelice

regress/expected/cypher_match.out

@@ -3652,7 +3652,7 @@ SELECT * FROM cypher('issue_2193', $$
            (b:Person {name: 'Tom', livesIn: 'Copenhagen'})
     WITH a, b
     MATCH (p:Person)
-    RETURN p.name
+    RETURN p.name ORDER BY p.name
 $$) AS (result agtype);
  result 
 --------
@@ -3677,7 +3677,7 @@ SELECT * FROM cypher('issue_2193', $$
     CREATE (a:City {name: 'Paris'})
     WITH a
     MATCH (c:City)
-    RETURN c.name
+    RETURN c.name ORDER BY c.name
 $$) AS (result agtype);
   result  
 ----------
@@ -3694,6 +3694,18 @@ $$) AS (result agtype);
 --------
 (0 rows)
 
+-- MATCH on non-existent label after DML predecessor still returns 0 rows
+-- and MATCH-introduced variable (p) is properly registered
+SELECT * FROM cypher('issue_2193', $$
+    CREATE (a:Person {name: 'Alice'})
+    WITH a
+    MATCH (p:NonExistentLabel)
+    RETURN p
+$$) AS (result agtype);
+ result 
+--------
+(0 rows)
+
 SELECT drop_graph('issue_2193', true);
 NOTICE:  drop cascades to 4 other objects
 DETAIL:  drop cascades to table issue_2193._ag_label_vertex

regress/sql/cypher_match.sql

@@ -1506,7 +1506,7 @@ SELECT * FROM cypher('issue_2193', $$
            (b:Person {name: 'Tom', livesIn: 'Copenhagen'})
     WITH a, b
     MATCH (p:Person)
-    RETURN p.name
+    RETURN p.name ORDER BY p.name
 $$) AS (result agtype);
 
 -- Single CREATE + MATCH on brand-new label
@@ -1522,7 +1522,7 @@ SELECT * FROM cypher('issue_2193', $$
     CREATE (a:City {name: 'Paris'})
     WITH a
     MATCH (c:City)
-    RETURN c.name
+    RETURN c.name ORDER BY c.name
 $$) AS (result agtype);
 
 -- MATCH on non-existent label without DML predecessor still returns 0 rows
@@ -1531,6 +1531,15 @@ SELECT * FROM cypher('issue_2193', $$
     RETURN x
 $$) AS (result agtype);
 
+-- MATCH on non-existent label after DML predecessor still returns 0 rows
+-- and MATCH-introduced variable (p) is properly registered
+SELECT * FROM cypher('issue_2193', $$
+    CREATE (a:Person {name: 'Alice'})
+    WITH a
+    MATCH (p:NonExistentLabel)
+    RETURN p
+$$) AS (result agtype);
+
 SELECT drop_graph('issue_2193', true);
 
 --

src/backend/parser/cypher_clause.c

@@ -2918,8 +2918,6 @@ static Query *transform_cypher_match_pattern(cypher_parsestate *cpstate,
     }
     else
     {
-        bool valid_labels = true;
-
         if (clause->prev)
         {
             RangeTblEntry *rte;
@@ -2971,21 +2969,21 @@ static Query *transform_cypher_match_pattern(cypher_parsestate *cpstate,
             if (clause_chain_has_dml(clause->prev) &&
                 !match_check_valid_label(self, cpstate))
             {
-                valid_labels = false;
+                cypher_bool_const *l = make_ag_node(cypher_bool_const);
+                cypher_bool_const *r = make_ag_node(cypher_bool_const);
+
+                l->boolean = true;
+                l->location = -1;
+                r->boolean = false;
+                r->location = -1;
+
+                where = (Node *)makeSimpleA_Expr(AEXPR_OP, "=",
+                                                 (Node *)l,
+                                                 (Node *)r, -1);
             }
         }
 
-        if (valid_labels)
-        {
-            transform_match_pattern(cpstate, query, self->pattern, where);
-        }
-        else
-        {
-            query->rtable = pstate->p_rtable;
-            query->rteperminfos = pstate->p_rteperminfos;
-            query->jointree = makeFromExpr(pstate->p_joinlist,
-                                           makeBoolConst(false, false));
-        }
+        transform_match_pattern(cpstate, query, self->pattern, where);
     }
 
     markTargetListOrigins(pstate, query->targetList);