Handle alias on IS NULL expressions in explain output

Add explainIsNullExprWithAlias to properly show aliases on IS NULL
expressions when wrapped in AliasedExpr.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: kyleconroy

internal/explain/expressions.go

@@ -808,6 +808,9 @@ func explainAliasedExpr(sb *strings.Builder, n *ast.AliasedExpr, depth int) {
 	case *ast.ExistsExpr:
 		// EXISTS expressions with alias
 		explainExistsExprWithAlias(sb, e, n.Alias, indent, depth)
+	case *ast.IsNullExpr:
+		// IS NULL expressions with alias
+		explainIsNullExprWithAlias(sb, e, n.Alias, indent, depth)
 	case *ast.Parameter:
 		// QueryParameter with alias
 		if e.Name != "" {

internal/explain/functions.go

@@ -1544,12 +1544,20 @@ func explainBetweenExprWithAlias(sb *strings.Builder, n *ast.BetweenExpr, alias
 }
 
 func explainIsNullExpr(sb *strings.Builder, n *ast.IsNullExpr, indent string, depth int) {
+	explainIsNullExprWithAlias(sb, n, "", indent, depth)
+}
+
+func explainIsNullExprWithAlias(sb *strings.Builder, n *ast.IsNullExpr, alias string, indent string, depth int) {
 	// IS NULL is represented as Function isNull
 	fnName := "isNull"
 	if n.Not {
 		fnName = "isNotNull"
 	}
-	fmt.Fprintf(sb, "%sFunction %s (children %d)\n", indent, fnName, 1)
+	if alias != "" {
+		fmt.Fprintf(sb, "%sFunction %s (alias %s) (children %d)\n", indent, fnName, alias, 1)
+	} else {
+		fmt.Fprintf(sb, "%sFunction %s (children %d)\n", indent, fnName, 1)
+	}
 	fmt.Fprintf(sb, "%s ExpressionList (children %d)\n", indent, 1)
 	Node(sb, n.Expr, depth+2)
 }

parser/testdata/03749_materialized_view_not_supports_parallel_write/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt8": true
-  }
-}
+{}