Merge pull request pingcap#541 from pingcap/shenli/issue-540

plans: Fix bug in show variables plan

plan/plans/show.go

@@ -378,10 +378,16 @@ func (s *ShowPlan) fetchShowVariables(ctx context.Context) error {
 
 		var value string
 		if !s.GlobalScope {
-			// Try to get Session Scope variable value
+			// Try to get Session Scope variable value first.
 			sv, ok := sessionVars.Systems[v.Name]
 			if ok {
 				value = sv
+			} else {
+				// If session scope variable is not set, get the global scope value.
+				value, err = ctx.(variable.GlobalSysVarAccessor).GetGlobalSysVar(ctx, v.Name)
+				if err != nil {
+					return errors.Trace(err)
+				}
 			}
 		} else {
 			value, err = ctx.(variable.GlobalSysVarAccessor).GetGlobalSysVar(ctx, v.Name)

plan/plans/show_test.go

@@ -180,6 +180,28 @@ func (p *testShowSuit) TestShowVariables(c *C) {
 	c.Assert(v, Equals, "on")
 }
 
+func (p *testShowSuit) TestIssue540(c *C) {
+	// Show variables where variable_name="time_zone"
+	pln := &plans.ShowPlan{
+		Target:      stmt.ShowVariables,
+		GlobalScope: false,
+		Pattern: &expression.PatternLike{
+			Pattern: &expression.Value{
+				Val: "time_zone",
+			},
+		},
+	}
+	// Make sure the session scope var is not set.
+	sessionVars := variable.GetSessionVars(p.ctx)
+	_, ok := sessionVars.Systems["time_zone"]
+	c.Assert(ok, IsFalse)
+
+	r, err := pln.Next(p.ctx)
+	c.Assert(err, IsNil)
+	c.Assert(r.Data[0], Equals, "time_zone")
+	c.Assert(r.Data[1], Equals, "SYSTEM")
+}
+
 func (p *testShowSuit) TestShowCollation(c *C) {
 	pln := &plans.ShowPlan{}