PlanBuilder: fix prepare from session value

executor/prepared_test.go

@@ -214,6 +214,15 @@ func (s *testSuite) TestPrepared(c *C) {
 		c.Assert(err, IsNil)
 		c.Assert(len(fields), Equals, 0)
 
+		// issue 8074
+		tk.MustExec("drop table if exists prepare1;")
+		tk.MustExec("create table prepare1 (a decimal(1))")
+		_, err = tk.Exec("prepare stmt FROM @sql1")
+		c.Assert(err.Error(), Equals, "line 1 column 4 near \"\" (total length 4)")
+		tk.MustExec("SET @sql = 'update prepare1 set a=5 where a=?';")
+		_, err = tk.Exec("prepare stmt FROM @sql")
+		c.Assert(err, IsNil)
+
 		// Coverage.
 		exec := &executor.ExecuteExec{}
 		exec.Next(ctx, nil)

planner/core/planbuilder.go

@@ -175,7 +175,7 @@ func (b *PlanBuilder) Build(node ast.Node) (Plan, error) {
 	case *ast.LoadStatsStmt:
 		return b.buildLoadStats(x), nil
 	case *ast.PrepareStmt:
-		return b.buildPrepare(x), nil
+		return b.buildPrepare(x)
 	case *ast.SelectStmt:
 		return b.buildSelect(x)
 	case *ast.UnionStmt:
@@ -396,17 +396,20 @@ func (b *PlanBuilder) buildSelectLock(src LogicalPlan, lock ast.SelectLockType)
 	return selectLock
 }
 
-func (b *PlanBuilder) buildPrepare(x *ast.PrepareStmt) Plan {
+func (b *PlanBuilder) buildPrepare(x *ast.PrepareStmt) (Plan, error) {
 	p := &Prepare{
 		Name: x.Name,
 	}
 	if x.SQLVar != nil {
-		// TODO: Prepared statement from variable expression do not work as expected.
-		// p.SQLText, _ = x.SQLVar.GetValue().(string)
+		if v, ok := b.ctx.GetSessionVars().Users[x.SQLVar.Name]; ok {
+			p.SQLText = v
+		} else {
+			p.SQLText = "NULL"
+		}
 	} else {
 		p.SQLText = x.SQLText
 	}
-	return p
+	return p, nil
 }
 
 func (b *PlanBuilder) buildCheckIndex(dbName model.CIStr, as *ast.AdminStmt) (Plan, error) {