Re: BUG #18536: Using WITH inside WITH RECURSIVE triggers a "shouldn't happen" error

I wrote:
> I think the case you show should be throwing
> ERROR:  recursive query "t" does not have the form non-recursive-term UNION [ALL] recursive-term

Hmm, that is probably too strong: it will break some queries we've
historically accepted.  What we need is just to forbid self-references
within the WITH clause.  The code actually does that already, it's
just doing it too late; so we can fix this with a simple re-ordering
of the error checks, as attached.

            regards, tom lane

diff --git a/src/backend/parser/parse_cte.c b/src/backend/parser/parse_cte.c
index 6826d4f36a..17432e9df6 100644
--- a/src/backend/parser/parse_cte.c
+++ b/src/backend/parser/parse_cte.c
@@ -877,6 +877,25 @@ checkWellFormedRecursion(CteState *cstate)
                             cte->ctename),
                      parser_errposition(cstate->pstate, cte->location)));

+        /*
+         * Really, we should insist that there not be a top-level WITH, since
+         * syntactically that would enclose the UNION.  However, we've not
+         * done so in the past and it's probably too late to change.  Settle
+         * for insisting that WITH not contain a self-reference.  Test this
+         * before examining the UNION arms, to avoid issuing confusing errors
+         * in such cases.
+         */
+        if (stmt->withClause)
+        {
+            cstate->curitem = i;
+            cstate->innerwiths = NIL;
+            cstate->selfrefcount = 0;
+            cstate->context = RECURSION_SUBLINK;
+            checkWellFormedRecursionWalker((Node *) stmt->withClause->ctes,
+                                           cstate);
+            Assert(cstate->innerwiths == NIL);
+        }
+
         /* The left-hand operand mustn't contain self-reference at all */
         cstate->curitem = i;
         cstate->innerwiths = NIL;
@@ -895,18 +914,6 @@ checkWellFormedRecursion(CteState *cstate)
         if (cstate->selfrefcount != 1)    /* shouldn't happen */
             elog(ERROR, "missing recursive reference");

-        /* WITH mustn't contain self-reference, either */
-        if (stmt->withClause)
-        {
-            cstate->curitem = i;
-            cstate->innerwiths = NIL;
-            cstate->selfrefcount = 0;
-            cstate->context = RECURSION_SUBLINK;
-            checkWellFormedRecursionWalker((Node *) stmt->withClause->ctes,
-                                           cstate);
-            Assert(cstate->innerwiths == NIL);
-        }
-
         /*
          * Disallow ORDER BY and similar decoration atop the UNION. These
          * don't make sense because it's impossible to figure out what they
diff --git a/src/test/regress/expected/with.out b/src/test/regress/expected/with.out
index b4f3121751..fff2d4949e 100644
--- a/src/test/regress/expected/with.out
+++ b/src/test/regress/expected/with.out
@@ -2029,6 +2029,38 @@ WITH RECURSIVE x(n) AS (SELECT n FROM x UNION ALL SELECT 1)
 ERROR:  recursive reference to query "x" must not appear within its non-recursive term
 LINE 1: WITH RECURSIVE x(n) AS (SELECT n FROM x UNION ALL SELECT 1)
                                               ^
+-- allow this, because we historically have
+WITH RECURSIVE x(n) AS (
+  WITH x1 AS (SELECT 1 AS n)
+    SELECT 0
+    UNION
+    SELECT * FROM x1)
+    SELECT * FROM x;
+ n
+---
+ 0
+ 1
+(2 rows)
+
+-- but this should be rejected
+WITH RECURSIVE x(n) AS (
+  WITH x1 AS (SELECT 1 FROM x)
+    SELECT 0
+    UNION
+    SELECT * FROM x1)
+    SELECT * FROM x;
+ERROR:  recursive reference to query "x" must not appear within a subquery
+LINE 2:   WITH x1 AS (SELECT 1 FROM x)
+                                    ^
+-- and this too
+WITH RECURSIVE x(n) AS (
+  (WITH x1 AS (SELECT 1 FROM x) SELECT * FROM x1)
+  UNION
+  SELECT 0)
+    SELECT * FROM x;
+ERROR:  recursive reference to query "x" must not appear within its non-recursive term
+LINE 2:   (WITH x1 AS (SELECT 1 FROM x) SELECT * FROM x1)
+                                     ^
 CREATE TEMPORARY TABLE y (a INTEGER);
 INSERT INTO y SELECT generate_series(1, 10);
 -- LEFT JOIN
diff --git a/src/test/regress/sql/with.sql b/src/test/regress/sql/with.sql
index 3fb0b33fce..06b2d4857f 100644
--- a/src/test/regress/sql/with.sql
+++ b/src/test/regress/sql/with.sql
@@ -908,6 +908,29 @@ WITH RECURSIVE x(n) AS (SELECT n FROM x)
 WITH RECURSIVE x(n) AS (SELECT n FROM x UNION ALL SELECT 1)
     SELECT * FROM x;

+-- allow this, because we historically have
+WITH RECURSIVE x(n) AS (
+  WITH x1 AS (SELECT 1 AS n)
+    SELECT 0
+    UNION
+    SELECT * FROM x1)
+    SELECT * FROM x;
+
+-- but this should be rejected
+WITH RECURSIVE x(n) AS (
+  WITH x1 AS (SELECT 1 FROM x)
+    SELECT 0
+    UNION
+    SELECT * FROM x1)
+    SELECT * FROM x;
+
+-- and this too
+WITH RECURSIVE x(n) AS (
+  (WITH x1 AS (SELECT 1 FROM x) SELECT * FROM x1)
+  UNION
+  SELECT 0)
+    SELECT * FROM x;
+
 CREATE TEMPORARY TABLE y (a INTEGER);
 INSERT INTO y SELECT generate_series(1, 10);