opt: allow casts in initial CTE expression

In #60560, we made the matching of types in UNIONs more strict. In the
recursive CTE code, we don't allow adding casts to the initial
expression, so the change caused us to regress in terms of supported
queries.

This change fixes this by allowing casts to the initial expression.
Not sure why I didn't allow this from the get-go.

Release note (bug fix): fixed "types cannot be matched for WITH
RECURSIVE" error in cases where we can cast the type in the initial
expression.

pkg/sql/logictest/testdata/logic_test/with

@@ -647,3 +647,30 @@ WITH cte AS (SELECT x*10+y FROM xy ORDER BY x+y LIMIT 3) SELECT * FROM cte
 11
 12
 21
+
+# Test for recursive CTE which needs a cast in the initial expression.
+statement ok
+CREATE TABLE graph_node (
+  id VARCHAR(16) PRIMARY KEY,
+  parent VARCHAR(16)
+)
+
+statement ok
+INSERT INTO graph_node (id, parent) VALUES
+  ('A', null),
+  ('B', 'A'),
+  ('C', 'B'),
+  ('D', 'C')
+
+query T rowsort
+WITH RECURSIVE nodes AS (
+  SELECT 'A' AS id
+  UNION ALL
+  SELECT graph_node.id FROM graph_node JOIN nodes ON graph_node.parent = nodes.id
+)
+SELECT * FROM nodes
+----
+A
+B
+C
+D

pkg/sql/opt/optbuilder/testdata/with

@@ -1327,11 +1327,95 @@ with &3 (cte)
       └── mapping:
            └──  a:5 => a:8
 
-# We don't support upcasting the "initial" query.
+# Verify the addition of casts to the "initial" query.
 build
 WITH RECURSIVE cte(x) AS (SELECT a FROM x UNION ALL SELECT x::FLOAT FROM cte WHERE x < 10) SELECT * FROM cte;
 ----
-error (42804): UNION types int and float cannot be matched for WITH RECURSIVE
+with &2 (cte)
+ ├── columns: x:9
+ ├── recursive-c-t-e
+ │    ├── columns: x:5
+ │    ├── working table binding: &1
+ │    ├── initial columns: a:8
+ │    ├── recursive columns: x:7
+ │    ├── fake-rel
+ │    │    └── columns: x.a:1
+ │    ├── project
+ │    │    ├── columns: a:8
+ │    │    ├── project
+ │    │    │    ├── columns: x.a:1
+ │    │    │    └── scan x
+ │    │    │         └── columns: x.a:1 b:2 rowid:3!null crdb_internal_mvcc_timestamp:4
+ │    │    └── projections
+ │    │         └── x.a:1::FLOAT8 [as=a:8]
+ │    └── project
+ │         ├── columns: x:7!null
+ │         ├── select
+ │         │    ├── columns: x:6!null
+ │         │    ├── with-scan &1 (cte)
+ │         │    │    ├── columns: x:6
+ │         │    │    └── mapping:
+ │         │    │         └──  x:5 => x:6
+ │         │    └── filters
+ │         │         └── x:6 < 10
+ │         └── projections
+ │              └── x:6::FLOAT8 [as=x:7]
+ └── with-scan &2 (cte)
+      ├── columns: x:9
+      └── mapping:
+           └──  x:5 => x:9
+
+exec-ddl
+CREATE TABLE graph_node (
+  id VARCHAR(16) PRIMARY KEY,
+  parent VARCHAR(16)
+)
+----
+
+# The 'A' in the initial query needs a cast to VARCHAR(16).
+build
+WITH RECURSIVE nodes AS (
+  SELECT 'A' AS id
+  UNION ALL
+  SELECT graph_node.id FROM graph_node JOIN nodes ON graph_node.parent = nodes.id
+)
+SELECT * FROM nodes;
+----
+with &2 (nodes)
+ ├── columns: id:8
+ ├── recursive-c-t-e
+ │    ├── columns: id:2
+ │    ├── working table binding: &1
+ │    ├── initial columns: id:7
+ │    ├── recursive columns: graph_node.id:3
+ │    ├── fake-rel
+ │    │    └── columns: id:1
+ │    ├── project
+ │    │    ├── columns: id:7!null
+ │    │    ├── project
+ │    │    │    ├── columns: id:1!null
+ │    │    │    ├── values
+ │    │    │    │    └── ()
+ │    │    │    └── projections
+ │    │    │         └── 'A' [as=id:1]
+ │    │    └── projections
+ │    │         └── id:1::VARCHAR(16) [as=id:7]
+ │    └── project
+ │         ├── columns: graph_node.id:3!null
+ │         └── inner-join (hash)
+ │              ├── columns: graph_node.id:3!null parent:4!null crdb_internal_mvcc_timestamp:5 id:6!null
+ │              ├── scan graph_node
+ │              │    └── columns: graph_node.id:3!null parent:4 crdb_internal_mvcc_timestamp:5
+ │              ├── with-scan &1 (nodes)
+ │              │    ├── columns: id:6
+ │              │    └── mapping:
+ │              │         └──  id:2 => id:6
+ │              └── filters
+ │                   └── parent:4 = id:6
+ └── with-scan &2 (nodes)
+      ├── columns: id:8
+      └── mapping:
+           └──  id:2 => id:8
 
 # Mutating WITHs not allowed at non-root positions.
 build

pkg/sql/opt/optbuilder/with.go

@@ -188,16 +188,7 @@ func (b *Builder) buildCTE(
 	// query.
 	outTypes, leftCastsNeeded, rightCastsNeeded := b.typeCheckSetOp(initialScope, recursiveScope, "UNION")
 	if leftCastsNeeded {
-		// We don't support casts on the initial expression; error out.
-		for i := range outTypes {
-			if !outTypes[i].Identical(initialScope.cols[i].typ) {
-				panic(pgerror.Newf(
-					pgcode.DatatypeMismatch,
-					"UNION types %s and %s cannot be matched for WITH RECURSIVE",
-					initialScope.cols[i].typ, recursiveScope.cols[i].typ,
-				))
-			}
-		}
+		initialScope = b.addCasts(initialScope, outTypes)
 	}
 	if rightCastsNeeded {
 		recursiveScope = b.addCasts(recursiveScope, outTypes)