planner: do not push keep descending order to tiflash (pingcap#13572)

cmd/explaintest/r/access_tiflash.result

@@ -40,3 +40,18 @@ id	count	task	operator info
 TableReader_7	44.00	root	data:Selection_6
 └─Selection_6	44.00	cop[tiflash]	or(and(gt(Column#1, 1), lt(Column#1, 20)), and(ge(Column#1, 30), lt(Column#1, 55)))
   └─TableScan_5	44.00	cop[tiflash]	table:tt, range:[-inf,+inf], keep order:false, stats:pseudo
+drop table if exists ttt;
+create table ttt (a int, primary key (a desc));
+desc select * from ttt order by ttt.a desc;
+id	count	task	operator info
+TableReader_11	10000.00	root	data:TableScan_10
+└─TableScan_10	10000.00	cop[tikv]	table:ttt, range:[-inf,+inf], keep order:true, desc, stats:pseudo
+desc select /*+ read_from_storage(tiflash[ttt]) */ * from ttt order by ttt.a desc;
+id	count	task	operator info
+Sort_4	10000.00	root	test.ttt.a:desc
+└─TableReader_8	10000.00	root	data:TableScan_7
+  └─TableScan_7	10000.00	cop[tiflash]	table:ttt, range:[-inf,+inf], keep order:false, stats:pseudo
+desc select /*+ read_from_storage(tiflash[ttt]) */ * from ttt order by ttt.a;
+id	count	task	operator info
+TableReader_11	10000.00	root	data:TableScan_10
+└─TableScan_10	10000.00	cop[tiflash]	table:ttt, range:[-inf,+inf], keep order:true, stats:pseudo

cmd/explaintest/t/access_tiflash.test

@@ -15,4 +15,11 @@ desc select /*+ read_from_storage(tiflash[t]) */ sum(isnull(a)) from t;
 create table tt(a int, b int, primary key(a));
 
 desc select * from tt where (tt.a > 1 and tt.a < 20) or (tt.a >= 30 and tt.a < 55);
-desc select /*+ read_from_storage(tiflash[tt]) */ * from tt where (tt.a > 1 and tt.a < 20) or (tt.a >= 30 and tt.a < 55);
+desc select /*+ read_from_storage(tiflash[tt]) */ * from tt where (tt.a > 1 and tt.a < 20) or (tt.a >= 30 and tt.a < 55);
+
+drop table if exists ttt;
+create table ttt (a int, primary key (a desc));
+
+desc select * from ttt order by ttt.a desc;
+desc select /*+ read_from_storage(tiflash[ttt]) */ * from ttt order by ttt.a desc;
+desc select /*+ read_from_storage(tiflash[ttt]) */ * from ttt order by ttt.a;

planner/core/find_best_task.go

@@ -280,7 +280,12 @@ func compareCandidates(lhs, rhs *candidatePath) int {
 func (ds *DataSource) getTableCandidate(path *accessPath, prop *property.PhysicalProperty) *candidatePath {
 	candidate := &candidatePath{path: path}
 	pkCol := ds.getPKIsHandleCol()
-	candidate.isMatchProp = len(prop.Items) == 1 && pkCol != nil && prop.Items[0].Col.Equal(nil, pkCol)
+	if len(prop.Items) == 1 && pkCol != nil {
+		candidate.isMatchProp = prop.Items[0].Col.Equal(nil, pkCol)
+		if path.storeType == kv.TiFlash {
+			candidate.isMatchProp = candidate.isMatchProp && !prop.Items[0].Desc
+		}
+	}
 	candidate.columnSet = expression.ExtractColumnSet(path.accessConds)
 	candidate.isSingleScan = true
 	return candidate
@@ -415,6 +420,12 @@ func (ds *DataSource) findBestTask(prop *property.PhysicalProperty) (t task, err
 			}, nil
 		}
 		if path.isTablePath {
+			if ds.preferStoreType&preferTiFlash != 0 && path.storeType == kv.TiKV {
+				continue
+			}
+			if ds.preferStoreType&preferTiKV != 0 && path.storeType == kv.TiFlash {
+				continue
+			}
 			tblTask, err := ds.convertToTableScan(prop, candidate)
 			if err != nil {
 				return nil, err
@@ -837,12 +848,6 @@ func (ds *DataSource) convertToTableScan(prop *property.PhysicalProperty, candid
 		filterCondition: path.tableFilters,
 		StoreType:       path.storeType,
 	}.Init(ds.ctx)
-	if ds.preferStoreType&preferTiFlash != 0 {
-		ts.StoreType = kv.TiFlash
-	}
-	if ds.preferStoreType&preferTiKV != 0 {
-		ts.StoreType = kv.TiKV
-	}
 	if ts.StoreType == kv.TiFlash {
 		ts.filterCondition = append(ts.filterCondition, ts.AccessCondition...)
 		ts.AccessCondition = nil

planner/core/logical_plan_builder.go

@@ -35,6 +35,7 @@ import (
 	"github.com/pingcap/tidb/expression"
 	"github.com/pingcap/tidb/expression/aggregation"
 	"github.com/pingcap/tidb/infoschema"
+	"github.com/pingcap/tidb/kv"
 	"github.com/pingcap/tidb/metrics"
 	"github.com/pingcap/tidb/planner/property"
 	"github.com/pingcap/tidb/privilege"
@@ -396,18 +397,30 @@ func (ds *DataSource) setPreferredStoreType(hintInfo *tableHintInfo) {
 	} else {
 		alias = &ds.tableInfo.Name
 	}
-	if hintInfo.ifPreferTiFlash(alias) {
-		ds.preferStoreType |= preferTiFlash
-	}
 	if hintInfo.ifPreferTiKV(alias) {
+		ds.preferStoreType |= preferTiKV
+	}
+	if hintInfo.ifPreferTiFlash(alias) {
 		if ds.preferStoreType != 0 {
 			errMsg := fmt.Sprintf("Storage hints are conflict, you can only specify one storage type of table %s", alias.L)
 			warning := ErrInternal.GenWithStack(errMsg)
 			ds.ctx.GetSessionVars().StmtCtx.AppendWarning(warning)
 			ds.preferStoreType = 0
 			return
 		}
-		ds.preferStoreType |= preferTiKV
+		ds.preferStoreType |= preferTiFlash
+		hasTiFlashPath := false
+		for _, path := range ds.possibleAccessPaths {
+			if path.storeType == kv.TiFlash {
+				hasTiFlashPath = true
+				break
+			}
+		}
+		// TODO: For now, if there is a TiFlash hint for a table, we enforce a TiFlash path. But hint is just a suggestion
+		//       for the planner. We can keep it since we need it to debug with PD and TiFlash. In future, this should be removed.
+		if !hasTiFlashPath {
+			ds.possibleAccessPaths = append(ds.possibleAccessPaths, &accessPath{isTablePath: true, storeType: kv.TiFlash})
+		}
 	}
 }