opt: better error message for unsupported UNION variant of recursive …

…CTEs

Postgres supports a variant of recursive CTEs that uses UNION instead
of UNION ALL. We don't support it as of yet; this commit improves the
error returned in this case.

Fixes #46378.

Release note: None

Release justification: low risk change to new functionality.

pkg/sql/opt/optbuilder/testdata/with

@@ -1143,6 +1143,40 @@ with &2 (cte)
            ├──  "?column?":7 => a:9
            └──  "?column?":8 => b:10
 
+# Allow non-recursive CTE even when it has UNION.
+build
+WITH RECURSIVE cte(a, b) AS (
+    SELECT 1, 2
+  UNION
+    SELECT 3, 4
+) SELECT * FROM cte;
+----
+with &2 (cte)
+ ├── columns: a:7!null b:8!null
+ ├── union
+ │    ├── columns: "?column?":5!null "?column?":6!null
+ │    ├── left columns: "?column?":1 "?column?":2
+ │    ├── right columns: "?column?":3 "?column?":4
+ │    ├── project
+ │    │    ├── columns: "?column?":1!null "?column?":2!null
+ │    │    ├── values
+ │    │    │    └── ()
+ │    │    └── projections
+ │    │         ├── 1 [as="?column?":1]
+ │    │         └── 2 [as="?column?":2]
+ │    └── project
+ │         ├── columns: "?column?":3!null "?column?":4!null
+ │         ├── values
+ │         │    └── ()
+ │         └── projections
+ │              ├── 3 [as="?column?":3]
+ │              └── 4 [as="?column?":4]
+ └── with-scan &2 (cte)
+      ├── columns: a:7!null b:8!null
+      └── mapping:
+           ├──  "?column?":5 => a:7
+           └──  "?column?":6 => b:8
+
 # Error cases.
 build
 WITH RECURSIVE cte(a, b) AS (
@@ -1158,7 +1192,7 @@ WITH RECURSIVE cte(a, b) AS (
     SELECT 1+a, 1+b FROM cte
 ) SELECT * FROM cte;
 ----
-error (42601): recursive query "cte" does not have the form non-recursive-term UNION ALL recursive-term
+error (0A000): unimplemented: recursive query "cte" uses UNION which is not implemented (only UNION ALL is supported)
 
 build
 WITH RECURSIVE cte(a, b) AS (

pkg/sql/opt/optbuilder/with.go

@@ -75,16 +75,25 @@ func (b *Builder) buildCTE(
 	}
 	cteScope.ctes = map[string]*cteSource{cte.Name.Alias.String(): cteSrc}
 
-	initial, recursive, ok := b.splitRecursiveCTE(cte.Stmt)
-	if !ok {
+	initial, recursive, isUnionAll, ok := b.splitRecursiveCTE(cte.Stmt)
+	// We don't currently support the UNION form (only UNION ALL).
+	if !ok || !isUnionAll {
 		// Build this as a non-recursive CTE, but throw a proper error message if it
 		// does have a recursive reference.
 		cteSrc.onRef = func() {
-			panic(pgerror.Newf(
-				pgcode.Syntax,
-				"recursive query %q does not have the form non-recursive-term UNION ALL recursive-term",
-				cte.Name.Alias,
-			))
+			if !ok {
+				panic(pgerror.Newf(
+					pgcode.Syntax,
+					"recursive query %q does not have the form non-recursive-term UNION ALL recursive-term",
+					cte.Name.Alias,
+				))
+			} else {
+				panic(unimplementedWithIssueDetailf(
+					46642, "",
+					"recursive query %q uses UNION which is not implemented (only UNION ALL is supported)",
+					cte.Name.Alias,
+				))
+			}
 		}
 		return b.buildCTE(cte, cteScope, false /* recursive */)
 	}
@@ -228,16 +237,16 @@ func (b *Builder) getCTECols(cteScope *scope, name tree.AliasClause) physical.Pr
 // returns ok=false.
 func (b *Builder) splitRecursiveCTE(
 	stmt tree.Statement,
-) (initial, recursive *tree.Select, ok bool) {
+) (initial, recursive *tree.Select, isUnionAll bool, ok bool) {
 	sel, ok := stmt.(*tree.Select)
 	// The form above doesn't allow for "outer" WITH, ORDER BY, or LIMIT
 	// clauses.
 	if !ok || sel.With != nil || sel.OrderBy != nil || sel.Limit != nil {
-		return nil, nil, false
+		return nil, nil, false, false
 	}
 	union, ok := sel.Select.(*tree.UnionClause)
-	if !ok || union.Type != tree.UnionOp || !union.All {
-		return nil, nil, false
+	if !ok || union.Type != tree.UnionOp {
+		return nil, nil, false, false
 	}
-	return union.Left, union.Right, true
+	return union.Left, union.Right, union.All, true
 }

pkg/util/errorutil/unimplemented/unimplemented.go

@@ -65,8 +65,7 @@ func NewWithIssueDetail(issue int, detail, msg string) error {
 	return unimplementedInternal(1 /*depth*/, issue, detail, false /*format*/, msg)
 }
 
-// NewWithIssueDetailf is like UnimplementedWithIssueDetail
-// but the message is formatted.
+// NewWithIssueDetailf is like NewWithIssueDetail but the message is formatted.
 func NewWithIssueDetailf(issue int, detail, format string, args ...interface{}) error {
 	return unimplementedInternal(1 /*depth*/, issue, detail, true /*format*/, format, args...)
 }