sql: add notice for Alter TYPE ADD VALUE IF NOT EXIST command

pkg/sql/alter_type.go

@@ -17,6 +17,7 @@ import (
 	"github.com/cockroachdb/cockroach/pkg/sql/catalog/descpb"
 	"github.com/cockroachdb/cockroach/pkg/sql/pgwire/pgcode"
 	"github.com/cockroachdb/cockroach/pkg/sql/pgwire/pgerror"
+	"github.com/cockroachdb/cockroach/pkg/sql/pgwire/pgnotice"
 	"github.com/cockroachdb/cockroach/pkg/sql/sem/tree"
 	"github.com/cockroachdb/cockroach/pkg/sql/sqlbase"
 	"github.com/cockroachdb/errors"
@@ -74,6 +75,23 @@ func (n *alterTypeNode) startExec(params runParams) error {
 func (p *planner) addEnumValue(
 	ctx context.Context, n *alterTypeNode, node *tree.AlterTypeAddValue,
 ) error {
+	if n.desc.Kind != descpb.TypeDescriptor_ENUM {
+		return pgerror.Newf(pgcode.WrongObjectType, "%q is not an enum", n.desc.Name)
+	}
+	// See if the value already exists in the enum or not.
+	for _, member := range n.desc.EnumMembers {
+		if member.LogicalRepresentation == node.NewVal {
+			if node.IfNotExists {
+				p.SendClientNotice(
+					ctx,
+					pgnotice.Newf("enum label %q already exists, skipping", node.NewVal),
+				)
+				return nil
+			}
+			return pgerror.Newf(pgcode.DuplicateObject, "enum label %q already exists", node.NewVal)
+		}
+	}
+
 	if err := n.desc.AddEnumValue(node); err != nil {
 		return err
 	}

pkg/sql/logictest/testdata/logic_test/notice

@@ -33,3 +33,18 @@ ALTER TABLE d.t RENAME TO d.t2
 ----
 NOTICE: renaming tables with a qualification is deprecated
 HINT: use ALTER TABLE d.t RENAME TO t2 instead
+
+statement ok
+SET experimental_enable_enums=true;
+
+# Start off with an empty enum, and add values to it.
+statement ok
+CREATE TYPE color AS ENUM ()
+
+statement ok
+ALTER TYPE color ADD VALUE 'black'
+
+query T noticetrace
+ALTER TYPE color ADD VALUE IF NOT EXISTS 'black'
+----
+NOTICE: enum label "black" already exists, skipping

pkg/sql/sqlbase/type_desc.go

@@ -254,25 +254,9 @@ func (desc *MutableTypeDescriptor) IsNew() bool {
 }
 
 // AddEnumValue adds an enum member to the type.
+// AddEnumValue assumes that the type is an enum, and that the new value
+// doesn't exist already in the enum.
 func (desc *MutableTypeDescriptor) AddEnumValue(node *tree.AlterTypeAddValue) error {
-	if desc.Kind != descpb.TypeDescriptor_ENUM {
-		return pgerror.Newf(pgcode.WrongObjectType, "%q is not an enum", desc.Name)
-	}
-	// See if the value already exists in the enum or not.
-	found := false
-	for _, member := range desc.EnumMembers {
-		if member.LogicalRepresentation == node.NewVal {
-			found = true
-			break
-		}
-	}
-	if found {
-		if node.IfNotExists {
-			return nil
-		}
-		return pgerror.Newf(pgcode.DuplicateObject, "enum label %q already exists", node.NewVal)
-	}
-
 	getPhysicalRep := func(idx int) []byte {
 		if idx < 0 || idx >= len(desc.EnumMembers) {
 			return nil