*: use terror instead of error

plan/plans/show.go

@@ -32,6 +32,7 @@ import (
 	"github.com/pingcap/tidb/sessionctx/variable"
 	"github.com/pingcap/tidb/stmt"
 	"github.com/pingcap/tidb/table"
+	"github.com/pingcap/tidb/terror"
 	"github.com/pingcap/tidb/util/charset"
 	"github.com/pingcap/tidb/util/format"
 )
@@ -414,7 +415,7 @@ func getSessionStatusVar(ctx context.Context, sessionVars *variable.SessionVars,
 	}
 
 	value, err := globalVars.GetGlobalStatusVar(ctx, name)
-	if err != nil && err != variable.ErrUnknownStatusVar {
+	if err != nil && terror.UnknownStatusVar.Equal(err) {
 		return "", errors.Trace(err)
 	}
 
@@ -429,7 +430,7 @@ func getGlobalStatusVar(ctx context.Context, sessionVars *variable.SessionVars,
 		return value, nil
 	}
 
-	if err != variable.ErrUnknownStatusVar {
+	if terror.UnknownStatusVar.Equal(err) {
 		return "", errors.Trace(err)
 	}
 

session.go

@@ -276,7 +276,7 @@ func (s *session) getExecRet(ctx context.Context, sql string) (string, error) {
 		return "", errors.Trace(err)
 	}
 	if row == nil {
-		return "", errors.New("result is empty")
+		return "", terror.ExecResultIsEmpty
 	}
 	value, err := types.ToString(row.Data[0])
 	if err != nil {
@@ -290,8 +290,7 @@ func (s *session) GetGlobalStatusVar(ctx context.Context, name string) (string,
 	// TODO: get global status variables from store.
 	v := variable.GetStatusVar(name)
 	if v == nil {
-		log.Errorf("get global status var %s is err:%v", name, variable.ErrUnknownStatusVar)
-		return "", variable.ErrUnknownStatusVar
+		return "", terror.UnknownStatusVar.Gen("unknown status variable:%s", name)
 	}
 
 	return v.Value, nil
@@ -302,8 +301,7 @@ func (s *session) SetGlobalStatusVar(ctx context.Context, name string, value str
 	// TODO: set global status variables from store.
 	v := variable.GetStatusVar(name)
 	if v == nil {
-		log.Errorf("set global status var %s is err:%v", name, variable.ErrUnknownStatusVar)
-		return variable.ErrUnknownStatusVar
+		return terror.UnknownStatusVar.Gen("unknown status variable:%s", name)
 	}
 	v.Value = value
 
@@ -315,7 +313,9 @@ func (s *session) GetGlobalSysVar(ctx context.Context, name string) (string, err
 	sql := fmt.Sprintf(`SELECT VARIABLE_VALUE FROM %s.%s WHERE VARIABLE_NAME="%s";`, mysql.SystemDB, mysql.GlobalVariablesTable, name)
 	sysVar, err := s.getExecRet(ctx, sql)
 	if err != nil {
-		log.Errorf("get global sys var %s is err:%v", name, err)
+		if terror.ExecResultIsEmpty.Equal(err) {
+			return "", terror.ExecResultIsEmpty.Gen("unknown sys variable:%s", name)
+		}
 		return "", errors.Trace(err)
 	}
 

terror/terror.go

@@ -28,6 +28,9 @@ var (
 
 	CommitNotInTransaction   = ClassExecutor.New(CodeCommitNotInTransaction, "commit not in transaction")
 	RollbackNotInTransaction = ClassExecutor.New(CodeRollbackNotInTransaction, "rollback not in transaction")
+	ExecResultIsEmpty        = ClassExecutor.New(CodeExecResultIsEmpty, "exec result is empty")
+
+	UnknownStatusVar = ClassVariable.New(CodeUnknownStatusVar, "unknown status variable")
 )
 
 // ErrCode represents a specific error type in a error class.
@@ -44,6 +47,7 @@ const (
 const (
 	CodeCommitNotInTransaction ErrCode = iota + 1
 	CodeRollbackNotInTransaction
+	CodeExecResultIsEmpty
 )
 
 // KV error codes.
@@ -52,6 +56,11 @@ const (
 	CodeNoDataForHandle
 )
 
+// Variable error codes.
+const (
+	CodeUnknownStatusVar ErrCode = iota + 1
+)
+
 // ErrClass represents a class of errors.
 type ErrClass int
 
@@ -63,6 +72,7 @@ const (
 	ClassExecutor
 	ClassKV
 	ClassServer
+	ClassVariable
 	// Add more as needed.
 )