schemadiff: DROP COLUMN not eligible for INSTANT algorithm if c…

…overed by an index (#15714)

Signed-off-by: Shlomi Noach <[email protected]>

go/vt/schemadiff/capability.go

@@ -21,6 +21,16 @@ func alterOptionCapableOfInstantDDL(alterOption sqlparser.AlterOption, createTab
 		}
 		return nil
 	}
+	findIndexCoveringColumn := func(colName string) *sqlparser.IndexDefinition {
+		for _, index := range createTable.TableSpec.Indexes {
+			for _, col := range index.Columns {
+				if col.Column.String() == colName {
+					return index
+				}
+			}
+		}
+		return nil
+	}
 	findTableOption := func(optName string) *sqlparser.TableOption {
 		if createTable == nil {
 			return nil
@@ -90,6 +100,10 @@ func alterOptionCapableOfInstantDDL(alterOption sqlparser.AlterOption, createTab
 				return false, nil
 			}
 		}
+		if findIndexCoveringColumn(opt.Name.Name.String()) != nil {
+			// not supported if the column is part of an index
+			return false, nil
+		}
 		if isVirtualColumn(opt.Name.Name.String()) {
 			// supported by all 8.0 versions
 			return capableOf(capabilities.InstantAddDropVirtualColumnFlavorCapability)

go/vt/schemadiff/capability_test.go

@@ -85,6 +85,18 @@ func TestAlterTableCapableOfInstantDDL(t *testing.T) {
 			alter:                     "alter table t drop column i1",
 			expectCapableOfInstantDDL: false,
 		},
+		{
+			name:                      "drop column fail due to index",
+			create:                    "create table t(id int, i1 int not null, i2 int not null, primary key(id), key i1_idx (i1))",
+			alter:                     "alter table t drop column i1",
+			expectCapableOfInstantDDL: false,
+		},
+		{
+			name:                      "drop column fail due to multicolumn index",
+			create:                    "create table t(id int, i1 int not null, i2 int not null, primary key(id), key i21_idx (i2, i1))",
+			alter:                     "alter table t drop column i1",
+			expectCapableOfInstantDDL: false,
+		},
 		{
 			name:                      "add two columns",
 			create:                    "create table t(id int, i1 int not null, primary key(id))",