fix

Author: James Cor

enginetest/queries/index_queries.go

@@ -4387,7 +4387,13 @@ var IndexQueries = []ScriptTest{
 			},
 			{
 				Query:       "alter table t drop index notanidx",
-				ExpectedErr: sql.ErrDuplicateKey,
+				ExpectedErr: sql.ErrCantDropFieldOrKey,
+			},
+			{
+				Query: "alter table t drop index if exists notanidx",
+				Expected: []sql.Row{
+					{types.NewOkResult(0)},
+				},
 			},
 		},
 	},

sql/rowexec/ddl_iters.go

@@ -2123,6 +2123,9 @@ func (b *BaseBuilder) executeAlterIndex(ctx *sql.Context, n *plan.AlterIndex) er
 		}
 		err = idxAltTbl.DropIndex(ctx, n.IndexName)
 		if err != nil {
+			if sql.ErrIndexNotFound.Is(err) && n.IfExists {
+				return nil
+			}
 			return err
 		}
 		return nil