evaluator: avoid evaluate independent subquery multiple times. (pingc…

…ap#1144)

* evaluator: avoid evaluate independent subquery multiple times.

* ast: rename UseOuterContext to Correlated.

ast/expressions.go

@@ -250,10 +250,10 @@ type SubqueryExec interface {
 type SubqueryExpr struct {
 	exprNode
 	// Query is the query SelectNode.
-	Query           ResultSetNode
-	SubqueryExec    SubqueryExec
-	Evaluated       bool
-	UseOuterContext bool
+	Query        ResultSetNode
+	SubqueryExec SubqueryExec
+	Evaluated    bool
+	Correlated   bool
 }
 
 // Accept implements Node Accept interface.

evaluator/evaluator.go

@@ -86,12 +86,7 @@ type Evaluator struct {
 
 // Enter implements ast.Visitor interface.
 func (e *Evaluator) Enter(in ast.Node) (out ast.Node, skipChildren bool) {
-	switch v := in.(type) {
-	case *ast.SubqueryExpr:
-		if v.Evaluated && !v.UseOuterContext {
-			// Subquery do not use outer context should only evaluate once.
-			return in, true
-		}
+	switch in.(type) {
 	case *ast.PatternInExpr, *ast.CompareSubqueryExpr:
 		e.multipleRows = true
 	case *ast.ExistsSubqueryExpr:
@@ -331,6 +326,10 @@ func (e *Evaluator) existsSubquery(v *ast.ExistsSubqueryExpr) bool {
 // Evaluate SubqueryExpr.
 // Get the value from v.SubQuery and set it to v.
 func (e *Evaluator) subqueryExpr(v *ast.SubqueryExpr) bool {
+	if v.Evaluated && !v.Correlated {
+		// Subquery do not use outer context should only evaluate once.
+		return true
+	}
 	err := EvalSubquery(e.ctx, v, e.multipleRows, e.existRow)
 	if err != nil {
 		e.err = errors.Trace(err)

executor/adapter.go

@@ -140,7 +140,7 @@ func (e *subqueryExecutor) Leave(in ast.Node) (ast.Node, bool) {
 	case *ast.ExistsSubqueryExpr:
 		e.existRow = false
 	case *ast.SubqueryExpr:
-		if !x.UseOuterContext {
+		if !x.Correlated {
 			err := evaluator.EvalSubquery(e.ctx, x, e.multipleRows, e.existRow)
 			if err != nil {
 				e.err = errors.Trace(err)

optimizer/resolver.go

@@ -289,7 +289,7 @@ func (nr *nameResolver) Leave(inNode ast.Node) (node ast.Node, ok bool) {
 		if nr.useOuterContext {
 			// TODO: check this
 			// If there is a deep nest of subquery, there may be something wrong.
-			v.UseOuterContext = true
+			v.Correlated = true
 			nr.useOuterContext = false
 		}
 	case *ast.TruncateTableStmt: