*: add cbo switch. (#3877)

plan/plan.go

@@ -30,7 +30,7 @@ import (
 
 // UseDAGPlanBuilder checks if we use new DAG planner.
 func UseDAGPlanBuilder(ctx context.Context) bool {
-	return ctx.GetClient().IsRequestTypeSupported(kv.ReqTypeDAG, kv.ReqSubTypeBasic)
+	return ctx.GetClient().IsRequestTypeSupported(kv.ReqTypeDAG, kv.ReqSubTypeBasic) && ctx.GetSessionVars().CBO
 }
 
 // Plan is the description of an execution flow.

session.go

@@ -1029,6 +1029,7 @@ const loadCommonGlobalVarsSQL = "select * from mysql.global_variables where vari
 	variable.TiDBIndexLookupConcurrency + quoteCommaQuote +
 	variable.TiDBIndexSerialScanConcurrency + quoteCommaQuote +
 	variable.TiDBMaxRowCountForINLJ + quoteCommaQuote +
+	variable.TiDBCBO + quoteCommaQuote +
 	variable.TiDBDistSQLScanConcurrency + "')"
 
 // loadCommonGlobalVariablesIfNeeded loads and applies commonly used global variables for the session.

sessionctx/variable/session.go

@@ -208,6 +208,9 @@ type SessionVars struct {
 
 	// MaxRowCountForINLJ defines max row count that the outer table of index nested loop join could be without force hint.
 	MaxRowCountForINLJ int
+
+	// CBO indicates if we use new planner with cbo.
+	CBO bool
 }
 
 // NewSessionVars creates a session vars object.
@@ -229,6 +232,7 @@ func NewSessionVars() *SessionVars {
 		IndexSerialScanConcurrency: DefIndexSerialScanConcurrency,
 		DistSQLScanConcurrency:     DefDistSQLScanConcurrency,
 		MaxRowCountForINLJ:         DefMaxRowCountForINLJ,
+		CBO:                        true,
 	}
 }
 

sessionctx/variable/sysvar.go

@@ -607,6 +607,7 @@ var defaultSysVars = []*SysVar{
 	{ScopeGlobal | ScopeSession, TiDBIndexLookupConcurrency, strconv.Itoa(DefIndexLookupConcurrency)},
 	{ScopeGlobal | ScopeSession, TiDBIndexSerialScanConcurrency, strconv.Itoa(DefIndexSerialScanConcurrency)},
 	{ScopeGlobal | ScopeSession, TiDBMaxRowCountForINLJ, strconv.Itoa(DefMaxRowCountForINLJ)},
+	{ScopeGlobal | ScopeSession, TiDBCBO, "ON"},
 	{ScopeGlobal | ScopeSession, TiDBSkipUTF8Check, boolToIntStr(DefSkipUTF8Check)},
 	{ScopeSession, TiDBBatchInsert, boolToIntStr(DefBatchInsert)},
 	{ScopeSession, TiDBCurrentTS, strconv.Itoa(DefCurretTS)},

sessionctx/variable/tidb_vars.go

@@ -90,6 +90,9 @@ const (
 	// It controls the max row count of outer table when do index nested loop join without hint.
 	// After the row count of the inner table is accurate, this variable will be removed.
 	TiDBMaxRowCountForINLJ = "tidb_max_row_count_for_inlj"
+
+	// tidb_cbo uses new planner with cost based optimizer.
+	TiDBCBO = "tidb_cbo"
 )
 
 // Default TiDB system variable values.

sessionctx/varsutil/varsutil.go

@@ -144,6 +144,8 @@ func SetSessionSystemVar(vars *variable.SessionVars, name string, value types.Da
 		vars.BatchInsert = tidbOptOn(sVal)
 	case variable.TiDBMaxRowCountForINLJ:
 		vars.MaxRowCountForINLJ = tidbOptPositiveInt(sVal, variable.DefMaxRowCountForINLJ)
+	case variable.TiDBCBO:
+		vars.CBO = tidbOptOn(sVal)
 	case variable.TiDBCurrentTS:
 		return variable.ErrReadOnly
 	}

store/tikv/coprocessor.go

@@ -45,7 +45,7 @@ func (c *CopClient) IsRequestTypeSupported(reqType, subType int64) bool {
 			return supportExpr(tipb.ExprType(subType))
 		}
 	case kv.ReqTypeDAG:
-		return c.store.mock
+		return true
 	}
 	return false
 }