fix: show tables to use any keyspace on system schema (#16521)

Signed-off-by: Harshit Gangal <harshit@planetscale.com>

go/test/endtoend/vtgate/misc_test.go

@@ -95,10 +95,13 @@ func TestShowTables(t *testing.T) {
 	conn, closer := start(t)
 	defer closer()
 
-	query := "show tables;"
-	qr := utils.Exec(t, conn, query)
-
+	qr := utils.Exec(t, conn, "show tables")
 	assert.Equal(t, "Tables_in_ks", qr.Fields[0].Name)
+
+	// no error on executing `show tables` on system schema
+	utils.Exec(t, conn, `use mysql`)
+	utils.Exec(t, conn, "show tables")
+	utils.Exec(t, conn, "show tables from information_schema")
 }
 
 func TestCastConvert(t *testing.T) {

go/vt/vtgate/executor_test.go

@@ -1105,6 +1105,15 @@ func TestExecutorShowTargeted(t *testing.T) {
 	}
 }
 
+func TestExecutorShowFromSystemSchema(t *testing.T) {
+	executor, _, _, _, ctx := createExecutorEnv(t)
+
+	session := NewSafeSession(&vtgatepb.Session{TargetString: "mysql"})
+
+	_, err := executor.Execute(ctx, nil, "TestExecutorShowFromSystemSchema", session, "show tables", nil)
+	require.NoError(t, err)
+}
+
 func TestExecutorUse(t *testing.T) {
 	executor, _, _, _, ctx := createExecutorEnv(t)
 

go/vt/vtgate/vcursor_impl.go

@@ -819,7 +819,7 @@ func commentedShardQueries(shardQueries []*querypb.BoundQuery, marginComments sq
 
 // TargetDestination implements the ContextVSchema interface
 func (vc *vcursorImpl) TargetDestination(qualifier string) (key.Destination, *vindexes.Keyspace, topodatapb.TabletType, error) {
-	keyspaceName := vc.keyspace
+	keyspaceName := vc.getActualKeyspace()
 	if vc.destination == nil && qualifier != "" {
 		keyspaceName = qualifier
 	}