Fix empty tuple and WITH clause expression parsing

1. Fix WITH clause to only treat ( as subquery when followed by SELECT/WITH
   Previously (SELECT...) AS name was parsed even for empty () or expressions

2. Fix empty tuple EXPLAIN output to use Function tuple format
   Empty tuples should output "Function tuple (children 1)" with ExpressionList
   rather than "Literal Tuple_()"

Fixes 41 tests including materialized views and WITH clause tests.

Author: claude

internal/explain/expressions.go

@@ -422,14 +422,18 @@ func explainAliasedExpr(sb *strings.Builder, n *ast.AliasedExpr, depth int) {
 		// Check if this is a tuple with complex expressions that should be rendered as Function tuple
 		if e.Type == ast.LiteralTuple {
 			if exprs, ok := e.Value.([]ast.Expression); ok {
-				hasComplexExpr := false
+				needsFunctionFormat := false
+				// Empty tuples always use Function tuple format
+				if len(exprs) == 0 {
+					needsFunctionFormat = true
+				}
 				for _, expr := range exprs {
 					if _, isLit := expr.(*ast.Literal); !isLit {
-						hasComplexExpr = true
+						needsFunctionFormat = true
 						break
 					}
 				}
-				if hasComplexExpr {
+				if needsFunctionFormat {
 					// Render as Function tuple with alias
 					fmt.Fprintf(sb, "%sFunction tuple (alias %s) (children %d)\n", indent, escapeAlias(n.Alias), 1)
 					fmt.Fprintf(sb, "%s ExpressionList (children %d)\n", indent, len(exprs))
@@ -683,6 +687,18 @@ func explainWithElement(sb *strings.Builder, n *ast.WithElement, indent string,
 	// When name is empty, don't show the alias part
 	switch e := n.Query.(type) {
 	case *ast.Literal:
+		// Empty tuples should be rendered as Function tuple, not Literal
+		if e.Type == ast.LiteralTuple {
+			if exprs, ok := e.Value.([]ast.Expression); ok && len(exprs) == 0 {
+				if n.Name != "" {
+					fmt.Fprintf(sb, "%sFunction tuple (alias %s) (children %d)\n", indent, n.Name, 1)
+				} else {
+					fmt.Fprintf(sb, "%sFunction tuple (children %d)\n", indent, 1)
+				}
+				fmt.Fprintf(sb, "%s ExpressionList\n", indent)
+				return
+			}
+		}
 		if n.Name != "" {
 			fmt.Fprintf(sb, "%sLiteral %s (alias %s)\n", indent, FormatLiteral(e), n.Name)
 		} else {

parser/parser.go

@@ -819,8 +819,8 @@ func (p *Parser) parseWithClause() []ast.Expression {
 				elem.Name = name
 				elem.Query = &ast.Identifier{Position: pos, Parts: []string{name}}
 			}
-		} else if p.currentIs(token.LPAREN) {
-			// Subquery: (SELECT ...) AS name
+		} else if p.currentIs(token.LPAREN) && (p.peekIs(token.SELECT) || p.peekIs(token.WITH)) {
+			// Subquery: (SELECT ...) AS name or (WITH ... SELECT ...) AS name
 			// In this syntax, the alias goes on the Subquery, not on WithElement
 			p.nextToken()
 			subquery := p.parseSelectWithUnion()

parser/testdata/00101_materialized_views_and_insert_without_explicit_database/metadata.json

@@ -3,8 +3,6 @@
     "stmt35": true,
     "stmt36": true,
     "stmt37": true,
-    "stmt38": true,
-    "stmt6": true,
-    "stmt7": true
+    "stmt38": true
   }
 }

parser/testdata/00148_summing_merge_tree_aggregate_function/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt78": true
-  }
-}
+{}

parser/testdata/00165_jit_aggregate_functions/metadata.json

@@ -1,6 +1 @@
-{
-  "explain_todo": {
-    "stmt10": true,
-    "stmt16": true
-  }
-}
+{}

parser/testdata/00172_constexprs_in_set/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt7": true
-  }
-}
+{}

parser/testdata/00175_partition_by_ignore/metadata.json

@@ -1,6 +1 @@
-{
-  "explain_todo": {
-    "stmt5": true,
-    "stmt6": true
-  }
-}
+{}

parser/testdata/00542_materialized_view_and_time_zone_tag/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt6": true
-  }
-}
+{}

parser/testdata/00610_materialized_view_forward_alter_partition_statements/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt5": true
-  }
-}
+{}

parser/testdata/00687_insert_into_mv/metadata.json

@@ -1,7 +1 @@
-{
-  "explain_todo": {
-    "stmt6": true,
-    "stmt7": true,
-    "stmt8": true
-  }
-}
+{}