Remove incorrect UNION ALL simplification in EXPLAIN output

.github/workflows/test.yml

@@ -21,4 +21,4 @@ jobs:
         run: go build ./...
 
       - name: Test
-        run: go test -v ./...
+        run: go test ./...

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
-  }
-}
+{}