executor: disable paging for small limit

distsql/request_builder.go

@@ -159,6 +159,7 @@ func (builder *RequestBuilder) SetDAGRequest(dag *tipb.DAGRequest) *RequestBuild
  if limit != nil && limit.Limit < estimatedRegionRowCount {
  builder.Request.Concurrency = 1
  }
+ builder.Request.LimitSize = limit.GetLimit()
  }
  return builder
 }

distsql/request_builder_test.go

@@ -676,6 +676,7 @@ func TestScanLimitConcurrency(t *testing.T) {
  Build()
  require.NoError(t, err)
  require.Equal(t, tt.concurrency, actual.Concurrency)
+ require.Equal(t, actual.LimitSize, tt.limit)
  })
  }
 }

kv/kv.go

@@ -573,6 +573,8 @@ type Request struct {
  StoreBatchSize int
  // ResourceGroupName is the name of the bind resource group.
  ResourceGroupName string
+ // LimitSize indicates whether the request is scan and limit
+ LimitSize uint64
 }
 
 // CoprRequestAdjuster is used to check and adjust a copr request according to specific rules.

store/copr/coprocessor.go

@@ -402,7 +402,13 @@ func buildCopTasks(bo *Backoffer, ranges *KeyRanges, opt *buildCopTaskOpt) ([]*c
  }
  i = nextI
  if req.Paging.Enable {
- pagingSize = paging.GrowPagingSize(pagingSize, req.Paging.MaxPagingSize)
+ if req.LimitSize != 0 && req.LimitSize < pagingSize {
+ // disable paging for small limit.
+ task.paging = false
+ task.pagingSize = 0
+ } else {
+ pagingSize = paging.GrowPagingSize(pagingSize, req.Paging.MaxPagingSize)
+ }
  }
  }
  }

store/copr/coprocessor_test.go

@@ -514,8 +514,6 @@ func TestBuildPagingTasks(t *testing.T) {
  req := &kv.Request{}
  req.Paging.Enable = true
  req.Paging.MinPagingSize = paging.MinPagingSize
- flashReq := &kv.Request{}
- flashReq.StoreType = kv.TiFlash
  tasks, err := buildTestCopTasks(bo, cache, buildCopRanges("a", "c"), req, nil)
  require.NoError(t, err)
  require.Len(t, tasks, 1)
@@ -525,6 +523,37 @@ func TestBuildPagingTasks(t *testing.T) {
  require.Equal(t, tasks[0].pagingSize, paging.MinPagingSize)
 }
 
+func TestBuildPagingTasksDisablePagingForSmallLimit(t *testing.T) {
+ mockClient, cluster, pdClient, err := testutils.NewMockTiKV("", nil)
+ require.NoError(t, err)
+ defer func() {
+ pdClient.Close()
+ err = mockClient.Close()
+ require.NoError(t, err)
+ }()
+ _, regionIDs, _ := testutils.BootstrapWithMultiRegions(cluster, []byte("g"), []byte("n"), []byte("t"))
+
+ pdCli := tikv.NewCodecPDClient(tikv.ModeTxn, pdClient)
+ defer pdCli.Close()
+
+ cache := NewRegionCache(tikv.NewRegionCache(pdCli))
+ defer cache.Close()
+
+ bo := backoff.NewBackofferWithVars(context.Background(), 3000, nil)
+
+ req := &kv.Request{}
+ req.Paging.Enable = true
+ req.Paging.MinPagingSize = paging.MinPagingSize
+ req.LimitSize = 1
+ tasks, err := buildTestCopTasks(bo, cache, buildCopRanges("a", "c"), req, nil)
+ require.NoError(t, err)
+ require.Len(t, tasks, 1)
+ require.Len(t, tasks, 1)
+ taskEqual(t, tasks[0], regionIDs[0], 0, "a", "c")
+ require.False(t, tasks[0].paging)
+ require.Equal(t, tasks[0].pagingSize, uint64(0))
+}
+
 func toCopRange(r kv.KeyRange) *coprocessor.KeyRange {
  coprRange := coprocessor.KeyRange{}
  coprRange.Start = r.StartKey