Revert "executor: fix show global variables return session variables …

…also (pingcap#19341) (pingcap#21045)" (pingcap#26240)

executor/show.go

@@ -667,36 +667,46 @@ func (e *ShowExec) fetchShowMasterStatus() error {
 
 func (e *ShowExec) fetchShowVariables() (err error) {
 	var (
-		value       string
-		sessionVars = e.ctx.GetSessionVars()
+		value         string
+		ok            bool
+		sessionVars   = e.ctx.GetSessionVars()
+		unreachedVars = make([]string, 0, len(variable.SysVars))
 	)
-	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})
-			}
-		}
-		return nil
-	}
-
-	// 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 !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
+		}
 		e.appendRow([]interface{}{v.Name, value})
 	}
+	if len(unreachedVars) != 0 {
+		systemVars, err := sessionVars.GlobalVarsAccessor.GetAllSysVars()
+		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})
+		}
+	}
 	return nil
 }
 

executor/show_test.go

@@ -29,7 +29,6 @@ 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"
@@ -1029,26 +1028,3 @@ 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)
-		}
-	}
-}