From 0ac0e154d957b519796f1088b0bc9f9ba5c2e639 Mon Sep 17 00:00:00 2001
From: Andrew <AndrewDi@users.noreply.github.com>
Date: Tue, 27 Nov 2018 11:00:41 +0800
Subject: [PATCH] planner, executor: fix `PREPARE FROM @var_name` (#8437)

---
 executor/prepared_test.go   | 14 ++++++++++++++
 planner/core/planbuilder.go |  7 +++++--
 2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/executor/prepared_test.go b/executor/prepared_test.go
index 0338145656173..07e54ef4ff356 100644
--- a/executor/prepared_test.go
+++ b/executor/prepared_test.go
@@ -214,6 +214,20 @@ 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))")
+		tk.MustExec("insert into prepare1 values(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)
+		tk.MustExec("set @var=1;")
+		_, err = tk.Exec("execute stmt using @var")
+		c.Assert(err, IsNil)
+		tk.MustQuery("select a from prepare1;").Check(testkit.Rows("5"))
+
 		// Coverage.
 		exec := &executor.ExecuteExec{}
 		exec.Next(ctx, nil)
diff --git a/planner/core/planbuilder.go b/planner/core/planbuilder.go
index c64de57ec8847..abf67bf9ff21c 100644
--- a/planner/core/planbuilder.go
+++ b/planner/core/planbuilder.go
@@ -401,8 +401,11 @@ func (b *PlanBuilder) buildPrepare(x *ast.PrepareStmt) Plan {
 		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
 	}