addressed review comments

Signed-off-by: Harshit Gangal <harshit@planetscale.com>

go/vt/sqlparser/ast_funcs.go

@@ -2369,11 +2369,11 @@ func GetFirstSelect(selStmt TableStatement) (*Select, error) {
 	case *Select:
 		return node, nil
 	case *ValuesStatement:
-		return nil, fmt.Errorf("first table_reference is a VALUES")
+		return nil, vterrors.VT12001("first table_reference as VALUES")
 	case *Union:
 		return GetFirstSelect(node.Left)
 	}
-	panic("[BUG]: unknown type for SelectStatement")
+	return nil, vterrors.VT13001(fmt.Sprintf("unknown type for SelectStatement: %T", selStmt))
 }
 
 // GetAllSelects gets all the select statement s

go/vt/sqlparser/ast_rewriting.go

@@ -532,7 +532,7 @@ func (er *astRewriter) existsRewrite(cursor *Cursor, node *ExistsExpr) {
 	sel.GroupBy = nil
 }
 
-// rewriteDistinctableAggr removed Distinctable from Max and Min Aggregations as it does not impact the result. But, makes the plan simpler.
+// rewriteDistinctableAggr removed Distinct from Max and Min Aggregations as it does not impact the result. But, makes the plan simpler.
 func (er *astRewriter) rewriteDistinctableAggr(cursor *Cursor, node DistinctableAggr) {
 	if !node.IsDistinct() {
 		return

go/vt/sqlparser/parser.go

@@ -55,8 +55,7 @@ func yyParsePooled(yylex yyLexer) int {
 		*parser = zeroParser
 		parserPool.Put(parser)
 	}()
-	parse := parser.Parse(yylex)
-	return parse
+	return parser.Parse(yylex)
 }
 
 // Instructions for creating new types: If a type

go/vt/vtgate/planbuilder/operators/expressions.go

@@ -17,10 +17,7 @@ limitations under the License.
 package operators
 
 import (
-	"fmt"
-
 	"vitess.io/vitess/go/vt/sqlparser"
-	"vitess.io/vitess/go/vt/vterrors"
 	"vitess.io/vitess/go/vt/vtgate/planbuilder/plancontext"
 	"vitess.io/vitess/go/vt/vtgate/semantics"
 )
@@ -128,7 +125,7 @@ func simplifyPredicates(ctx *plancontext.PlanningContext, in sqlparser.Expr) sql
 func getFirstSelect(selStmt sqlparser.TableStatement) *sqlparser.Select {
 	firstSelect, err := sqlparser.GetFirstSelect(selStmt)
 	if err != nil {
-		panic(vterrors.VT12001(fmt.Sprintf("first UNION part not a SELECT: %v", err)))
+		panic(err)
 	}
 	return firstSelect
 }

go/vt/vtgate/planbuilder/operators/horizon_expanding.go

@@ -95,7 +95,7 @@ func expandSelectHorizon(ctx *plancontext.PlanningContext, horizon *Horizon, sel
 
 	if qp.NeedsDistinct() {
 		op = newDistinct(op, qp, true)
-		extracted = append(extracted, "Distinctable")
+		extracted = append(extracted, "Distinct")
 	}
 
 	if sel.Having != nil {

go/vt/vtgate/planbuilder/operators/phases.go

@@ -59,7 +59,7 @@ func (p Phase) String() string {
 	case addAggrOrdering:
 		return "optimize aggregations with ORDER BY"
 	case cleanOutPerfDistinct:
-		return "optimize Distinctable operations"
+		return "optimize Distinct operations"
 	case subquerySettling:
 		return "settle subqueries"
 	case dmlWithInput: