planner: allow read from TiKV for sys schema when `tidb_isolation_r…

…ead_engines` is set to tiflash (#51219)

close #51218

pkg/planner/core/planbuilder.go

@@ -1236,7 +1236,7 @@ func getPossibleAccessPaths(ctx PlanContext, tableHints *hint.PlanHints, indexHi
 
 func filterPathByIsolationRead(ctx PlanContext, paths []*util.AccessPath, tblName model.CIStr, dbName model.CIStr) ([]*util.AccessPath, error) {
 	// TODO: filter paths with isolation read locations.
-	if dbName.L == mysql.SystemDB {
+	if util2.IsSysDB(dbName.L) {
 		return paths, nil
 	}
 	isolationReadEngines := ctx.GetSessionVars().GetIsolationReadEngines()

tests/integrationtest/r/planner/core/integration.result

@@ -4352,3 +4352,12 @@ StreamAgg	1.00	root		group by:planner__core__integration.sbtest.k, funcs:count(1
   └─TableReader	1.00	root		data:TableRangeScan
     └─TableRangeScan	1.00	cop[tikv]	table:sbtest	range:[0,1), keep order:false, stats:pseudo
 set @@tidb_opt_fix_control = default;
+create table sys.t (id int);
+insert into sys.t values (1),(2);
+set tidb_isolation_read_engines='tiflash';
+select * from sys.t;
+id
+1
+2
+drop table sys.t;
+set tidb_isolation_read_engines=DEFAULT;

tests/integrationtest/t/planner/core/integration.test

@@ -2386,3 +2386,11 @@ set @@tidb_opt_fix_control = '46177:on';
 explain format='brief'  select row_number() over(order by a.k) from (select * from sbtest where id<10) a;
 explain format='brief' select /*+ stream_agg() */ count(1) from sbtest where id<1 group by k;
 set @@tidb_opt_fix_control = default;
+
+# TestSysIsAccessibleWithTiFlashReadIsolation
+create table sys.t (id int);
+insert into sys.t values (1),(2);
+set tidb_isolation_read_engines='tiflash';
+select * from sys.t;
+drop table sys.t;
+set tidb_isolation_read_engines=DEFAULT;