Remove incorrect UNION ALL simplification in EXPLAIN output

The simplifyUnionSelects function was incorrectly collapsing UNION ALL
queries when selects had identical expressions but different aliases.
ClickHouse's EXPLAIN AST output keeps all SELECT queries in a UNION,
so this optimization was incorrect.

Fixes test 00592_union_all_different_aliases.

Author: claude

internal/explain/select.go

@@ -248,98 +248,9 @@ func countSelectUnionChildren(n *ast.SelectWithUnionQuery) int {
 	return count
 }
 
-// simplifyUnionSelects implements ClickHouse's UNION ALL optimization:
-// When all SELECT queries in a UNION have identical expressions (ignoring aliases)
-// but different aliases, only the first SELECT is returned.
-// This only applies when ALL columns in ALL SELECTs have explicit aliases.
-// If aliases are the same across all SELECTs, or if any column lacks an alias, all are kept.
+// simplifyUnionSelects returns all SELECT statements in a UNION.
+// ClickHouse does not simplify UNION ALL queries in EXPLAIN AST output.
 func simplifyUnionSelects(selects []ast.Statement) []ast.Statement {
-	if len(selects) <= 1 {
-		return selects
-	}
-
-	// Check if all are simple SelectQuery with only literal columns
-	var queries []*ast.SelectQuery
-	for _, sel := range selects {
-		sq, ok := sel.(*ast.SelectQuery)
-		if !ok {
-			// Not a simple SelectQuery, can't simplify
-			return selects
-		}
-		// Only handle simple SELECT with just columns, no FROM/WHERE/etc.
-		if sq.From != nil || sq.Where != nil || sq.GroupBy != nil ||
-			sq.Having != nil || sq.OrderBy != nil || len(sq.With) > 0 {
-			return selects
-		}
-		queries = append(queries, sq)
-	}
-
-	// Check if all have the same number of columns
-	numCols := len(queries[0].Columns)
-	for _, q := range queries[1:] {
-		if len(q.Columns) != numCols {
-			return selects
-		}
-	}
-
-	// Check if columns are all literals with aliases
-	// and compare expressions (without aliases) and aliases separately
-	allSameAliases := true
-	allSameExprs := true
-	allHaveAliases := true
-
-	for colIdx := 0; colIdx < numCols; colIdx++ {
-		firstAlias := ""
-		firstExpr := ""
-
-		for i, q := range queries {
-			col := q.Columns[colIdx]
-			alias := ""
-			exprStr := ""
-			hasAlias := false
-
-			switch c := col.(type) {
-			case *ast.AliasedExpr:
-				alias = c.Alias
-				hasAlias = c.Alias != ""
-				// Get string representation of the expression
-				if lit, ok := c.Expr.(*ast.Literal); ok {
-					exprStr = fmt.Sprintf("%v", lit.Value)
-				} else {
-					// Non-literal expression, can't simplify
-					return selects
-				}
-			case *ast.Literal:
-				exprStr = fmt.Sprintf("%v", c.Value)
-				hasAlias = false
-			default:
-				// Not a simple literal or aliased literal
-				return selects
-			}
-
-			if !hasAlias {
-				allHaveAliases = false
-			}
-
-			if i == 0 {
-				firstAlias = alias
-				firstExpr = exprStr
-			} else {
-				if alias != firstAlias {
-					allSameAliases = false
-				}
-				if exprStr != firstExpr {
-					allSameExprs = false
-				}
-			}
-		}
-	}
-
-	// If expressions are the same, all have aliases, but aliases differ, return only first SELECT
-	if allSameExprs && allHaveAliases && !allSameAliases {
-		return selects[:1]
-	}
-
 	return selects
 }
 

parser/testdata/00592_union_all_different_aliases/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt1": true
-  }
-}
+{}