From f1aa21580838237f9f828d391070a9d24e65ce6f Mon Sep 17 00:00:00 2001
From: Ewan Chou <coocood@gmail.com>
Date: Wed, 18 Nov 2015 19:30:00 +0800
Subject: [PATCH] tidb: set session variable when load from global variable.

When autocommit is not set by session, we use global variable.
Currently it is loaded in every statement, which means we execute a select statement for every statements.
The behaviour not only hurts performance, but also is wrong.
In MySQL, autocommit is determined when session is created, and it will not be affected when global variable updates.

Also added a basic benchmark method to test basic performace test.
---
 bench_test.go | 30 ++++++++++++++++++++++++++++++
 session.go    |  1 +
 2 files changed, 31 insertions(+)
 create mode 100644 bench_test.go

diff --git a/bench_test.go b/bench_test.go
new file mode 100644
index 0000000000..1c29d54bc7
--- /dev/null
+++ b/bench_test.go
@@ -0,0 +1,30 @@
+package tidb
+
+import (
+	"testing"
+
+	"github.com/ngaut/log"
+)
+
+func BenchmarkBasic(b *testing.B) {
+	store, err := NewStore("memory://bench")
+	if err != nil {
+		b.Fatal(err)
+	}
+	log.SetLevel(log.LOG_LEVEL_ERROR)
+	se, err := CreateSession(store)
+	if err != nil {
+		b.Fatal(err)
+	}
+	for i := 0; i < b.N; i++ {
+		rs, err := se.Execute("select 1")
+		if err != nil {
+			b.Fatal(err)
+		}
+		row, err := rs[0].Next()
+		if err != nil || row == nil {
+			b.Fatal(err)
+		}
+		rs[0].Close()
+	}
+}
diff --git a/session.go b/session.go
index 64c7e57ca9..5906ef7abd 100644
--- a/session.go
+++ b/session.go
@@ -355,6 +355,7 @@ func (s *session) isAutocommit(ctx context.Context) bool {
 			log.Errorf("Get global sys var error: %v", err)
 			return false
 		}
+		variable.GetSessionVars(ctx).Systems["autocommit"] = autocommit
 		ok = true
 	}
 	if ok && (autocommit == "ON" || autocommit == "on" || autocommit == "1") {