executor: fix show global variables return session variables also (pi…

…ngcap#19341) (pingcap#21045)

Signed-off-by: ti-srebot <[email protected]>

executor/show.go

@@ -646,45 +646,35 @@ func (e *ShowExec) fetchShowMasterStatus() error {
 
 func (e *ShowExec) fetchShowVariables() (err error) {
 	var (
-		value         string
-		ok            bool
-		sessionVars   = e.ctx.GetSessionVars()
-		unreachedVars = make([]string, 0, len(variable.SysVars))
+		value       string
+		sessionVars = e.ctx.GetSessionVars()
 	)
-	for _, v := range variable.SysVars {
-		if !e.GlobalScope {
-			// For a session scope variable,
-			// 1. try to fetch value from SessionVars.Systems;
-			// 2. if this variable is session-only, fetch value from SysVars
-			//		otherwise, fetch the value from table `mysql.Global_Variables`.
-			value, ok, err = variable.GetSessionOnlySysVars(sessionVars, v.Name)
-		} else {
-			// If the scope of a system variable is ScopeNone,
-			// it's a read-only variable, so we return the default value of it.
-			// Otherwise, we have to fetch the values from table `mysql.Global_Variables` for global variable names.
-			value, ok, err = variable.GetScopeNoneSystemVar(v.Name)
-		}
-		if err != nil {
-			return errors.Trace(err)
-		}
-		if !ok {
-			unreachedVars = append(unreachedVars, v.Name)
-			continue
+	if e.GlobalScope {
+		// Collect global scope variables,
+		// 1. Exclude the variables of ScopeSession in variable.SysVars;
+		// 2. If the variable is ScopeNone, it's a read-only variable, return the default value of it,
+		// 		otherwise, fetch the value from table `mysql.Global_Variables`.
+		for _, v := range variable.SysVars {
+			if v.Scope != variable.ScopeSession {
+				value, err = variable.GetGlobalSystemVar(sessionVars, v.Name)
+				if err != nil {
+					return errors.Trace(err)
+				}
+				e.appendRow([]interface{}{v.Name, value})
+			}
 		}
-		e.appendRow([]interface{}{v.Name, value})
+		return nil
 	}
-	if len(unreachedVars) != 0 {
-		systemVars, err := sessionVars.GlobalVarsAccessor.GetAllSysVars()
+
+	// Collect session scope variables,
+	// If it is a session only variable, use the default value defined in code,
+	//   otherwise, fetch the value from table `mysql.Global_Variables`.
+	for _, v := range variable.SysVars {
+		value, err = variable.GetSessionSystemVar(sessionVars, v.Name)
 		if err != nil {
 			return errors.Trace(err)
 		}
-		for _, varName := range unreachedVars {
-			varValue, ok := systemVars[varName]
-			if !ok {
-				varValue = variable.SysVars[varName].Value
-			}
-			e.appendRow([]interface{}{varName, varValue})
-		}
+		e.appendRow([]interface{}{v.Name, value})
 	}
 	return nil
 }

executor/show_test.go

@@ -29,6 +29,7 @@ import (
 	"github.com/pingcap/tidb/privilege/privileges"
 	"github.com/pingcap/tidb/session"
 	"github.com/pingcap/tidb/sessionctx"
+	"github.com/pingcap/tidb/sessionctx/variable"
 	"github.com/pingcap/tidb/types"
 	"github.com/pingcap/tidb/util/testkit"
 	"github.com/pingcap/tidb/util/testutil"
@@ -962,3 +963,26 @@ func (s *testSuite5) TestShowClusterConfig(c *C) {
 	confErr = fmt.Errorf("something unknown error")
 	c.Assert(tk.QueryToErr("show config"), ErrorMatches, confErr.Error())
 }
+
+func (s *testSuite5) TestShowVar(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	var showSQL string
+	for _, v := range variable.SysVars {
+		// When ScopeSession only. `show global variables` must return empty.
+		if v.Scope == variable.ScopeSession {
+			showSQL = "show variables like '" + v.Name + "'"
+			res := tk.MustQuery(showSQL)
+			c.Check(res.Rows(), HasLen, 1)
+			showSQL = "show global variables like '" + v.Name + "'"
+			res = tk.MustQuery(showSQL)
+			c.Check(res.Rows(), HasLen, 0)
+		} else {
+			showSQL = "show global variables like '" + v.Name + "'"
+			res := tk.MustQuery(showSQL)
+			c.Check(res.Rows(), HasLen, 1)
+			showSQL = "show variables like '" + v.Name + "'"
+			res = tk.MustQuery(showSQL)
+			c.Check(res.Rows(), HasLen, 1)
+		}
+	}
+}