ddl: disallow dropping index on auto_increment column

ddl/db_integration_test.go

@@ -414,6 +414,8 @@ func (s *testIntegrationSuite5) TestMySQLErrorCode(c *C) {
 	assertErrorCode(c, tk, sql, tmysql.ErrPrimaryCantHaveNull)
 	sql = "create table t2 (id int null, age int, primary key(id));"
 	assertErrorCode(c, tk, sql, tmysql.ErrPrimaryCantHaveNull)
+	sql = "create table t2 (id int auto_increment);"
+	assertErrorCode(c, tk, sql, tmysql.ErrWrongAutoKey)
 
 	sql = "create table t2 (id int primary key , age int);"
 	tk.MustExec(sql)
@@ -1803,3 +1805,19 @@ func (s *testIntegrationSuite5) TestChangingDBCharset(c *C) {
 	tk.MustExec("ALTER SCHEMA CHARACTER SET = 'utf8mb4' COLLATE = 'utf8mb4_general_ci'")
 	verifyDBCharsetAndCollate("alterdb2", "utf8mb4", "utf8mb4_general_ci")
 }
+
+func (s *testIntegrationSuite4) TestDropAutoIncrementIndex(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("create database if not exists test")
+	tk.MustExec("use test")
+
+	tk.MustExec("drop table if exists t1")
+	tk.MustExec("create table t1 (a int auto_increment, unique key (a))")
+	dropIndexSQL := "alter table t1 drop index a"
+	assertErrorCode(c, tk, dropIndexSQL, mysql.ErrWrongAutoKey)
+
+	tk.MustExec("drop table if exists t1")
+	tk.MustExec("create table t1 (a int(11) not null auto_increment, b int(11), c bigint, unique key (a, b, c))")
+	dropIndexSQL = "alter table t1 drop index a"
+	assertErrorCode(c, tk, dropIndexSQL, mysql.ErrWrongAutoKey)
+}

ddl/ddl_api.go

@@ -3263,7 +3263,8 @@ func (d *ddl) DropIndex(ctx sessionctx.Context, ti ast.Ident, indexName model.CI
 		return errors.Trace(infoschema.ErrTableNotExists.GenWithStackByArgs(ti.Schema, ti.Name))
 	}
 
-	if indexInfo := t.Meta().FindIndexByName(indexName.L); indexInfo == nil {
+	indexInfo := t.Meta().FindIndexByName(indexName.L)
+	if indexInfo == nil {
 		err = ErrCantDropFieldOrKey.GenWithStack("index %s doesn't exist", indexName)
 		if ifExists {
 			ctx.GetSessionVars().StmtCtx.AppendNote(err)
@@ -3272,6 +3273,13 @@ func (d *ddl) DropIndex(ctx sessionctx.Context, ti ast.Ident, indexName model.CI
 		return err
 	}
 
+	cols := t.Cols()
+	for _, idxCol := range indexInfo.Columns {
+		if mysql.HasAutoIncrementFlag(cols[idxCol.Offset].Flag) {
+			return errors.Trace(autoid.ErrWrongAutoKey)
+		}
+	}
+
 	job := &model.Job{
 		SchemaID:   schema.ID,
 		TableID:    t.Meta().ID,

meta/autoid/errors.go

@@ -21,12 +21,14 @@ import (
 // Error instances.
 var (
 	ErrAutoincReadFailed = terror.ClassAutoid.New(mysql.ErrAutoincReadFailed, mysql.MySQLErrName[mysql.ErrAutoincReadFailed])
+	ErrWrongAutoKey      = terror.ClassAutoid.New(mysql.ErrWrongAutoKey, mysql.MySQLErrName[mysql.ErrWrongAutoKey])
 )
 
 func init() {
 	// Map error codes to mysql error codes.
 	tableMySQLErrCodes := map[terror.ErrCode]uint16{
 		mysql.ErrAutoincReadFailed: mysql.ErrAutoincReadFailed,
+		mysql.ErrWrongAutoKey:      mysql.ErrWrongAutoKey,
 	}
 	terror.ErrClassToMySQLCodes[terror.ClassAutoid] = tableMySQLErrCodes
 }

planner/core/preprocess.go

@@ -26,6 +26,7 @@ import (
 	"github.com/pingcap/tidb/ddl"
 	"github.com/pingcap/tidb/expression"
 	"github.com/pingcap/tidb/infoschema"
+	"github.com/pingcap/tidb/meta/autoid"
 	"github.com/pingcap/tidb/sessionctx"
 	"github.com/pingcap/tidb/types"
 	"github.com/pingcap/tidb/types/parser_driver"
@@ -295,7 +296,7 @@ func (p *preprocessor) checkAutoIncrement(stmt *ast.CreateTableStmt) {
 		}
 	}
 	if (autoIncrementMustBeKey && !isKey) || count > 1 {
-		p.err = errors.New("Incorrect table definition; there can be only one auto column and it must be defined as a key")
+		p.err = autoid.ErrWrongAutoKey.GenWithStackByArgs()
 	}
 
 	switch autoIncrementCol.Tp.Tp {

planner/core/preprocess_test.go

@@ -53,11 +53,11 @@ func (s *testValidatorSuite) TestValidator(c *C) {
 		{"create table t(id int auto_increment default null, primary key (id))", true, nil},
 		{"create table t(id int default null auto_increment, primary key (id))", true, nil},
 		{"create table t(id int not null auto_increment)", true,
-			errors.New("Incorrect table definition; there can be only one auto column and it must be defined as a key")},
+			errors.New("[autoid:1075]Incorrect table definition; there can be only one auto column and it must be defined as a key")},
 		{"create table t(id int not null auto_increment, c int auto_increment, key (id, c))", true,
-			errors.New("Incorrect table definition; there can be only one auto column and it must be defined as a key")},
+			errors.New("[autoid:1075]Incorrect table definition; there can be only one auto column and it must be defined as a key")},
 		{"create table t(id int not null auto_increment, c int, key (c, id))", true,
-			errors.New("Incorrect table definition; there can be only one auto column and it must be defined as a key")},
+			errors.New("[autoid:1075]Incorrect table definition; there can be only one auto column and it must be defined as a key")},
 		{"create table t(id decimal auto_increment, key (id))", true,
 			errors.New("Incorrect column specifier for column 'id'")},
 		{"create table t(id float auto_increment, key (id))", true, nil},