add UseIndex API

main.go

@@ -191,6 +191,11 @@ func (s *DB) SubQuery() *expr {
 	return Expr(fmt.Sprintf("(%v)", scope.SQL), scope.SQLVars...)
 }
 
+// UseIndex use a specified index when query
+func (s *DB) UseIndex(index string) *DB {
+	return s.clone().search.UseIndex(index).db
+}
+
 // Where return a new relation, filter records with given conditions, accepts `map`, `struct` or `string` as conditions, refer http://jinzhu.github.io/gorm/crud.html#query
 func (s *DB) Where(query interface{}, args ...interface{}) *DB {
 	return s.clone().search.Where(query, args...).db

scope.go

@@ -708,6 +708,10 @@ func (scope *Scope) buildSelectQuery(clause map[string]interface{}) (str string)
 	return
 }
 
+func (scope *Scope) useIndexSQL() string {
+	return fmt.Sprintf("USE INDEX(%s)", scope.Search.useIndex)
+}
+
 func (scope *Scope) whereSQL() (sql string) {
 	var (
 		quotedTableName                                = scope.QuotedTableName()

scope_test.go

@@ -10,7 +10,7 @@ import (
 )
 
 func NameIn1And2(d *gorm.DB) *gorm.DB {
-	return d.Where("name in (?)", []string{"ScopeUser1", "ScopeUser2"})
+	return d.UseIndex("PRIMARY").Where("name in (?)", []string{"ScopeUser1", "ScopeUser2"})
 }
 
 func NameIn2And3(d *gorm.DB) *gorm.DB {

search.go

@@ -21,6 +21,7 @@ type search struct {
 	limit            interface{}
 	group            string
 	tableName        string
+	useIndex         string
 	raw              bool
 	Unscoped         bool
 	ignoreOrderQuery bool
@@ -36,6 +37,15 @@ func (s *search) clone() *search {
 	return &clone
 }
 
+// UseIndex makes SQL use the specified index
+//
+// SELECT * FROM trades USE INDEX(index_trades_on_market_uuid) WHERE market_uuid="test market" ORDER BY id DESC;
+//
+func (s *search) UseIndex(index string) *search {
+	s.useIndex = index
+	return s
+}
+
 func (s *search) Where(query interface{}, values ...interface{}) *search {
 	s.whereConditions = append(s.whereConditions, map[string]interface{}{"query": query, "args": values})
 	return s