executor: try to optimize index lookup query may send too many cop ta…

…sk (#53855)

close #53871

pkg/executor/distsql.go

@@ -676,7 +676,6 @@ func (e *IndexLookUpExecutor) startIndexWorker(ctx context.Context, workCh chan<
 			SetStartTS(e.startTS).
 			SetDesc(e.desc).
 			SetKeepOrder(e.keepOrder).
-			SetPaging(e.indexPaging).
 			SetTxnScope(e.txnScope).
 			SetReadReplicaScope(e.readReplicaScope).
 			SetIsStaleness(e.isStaleness).
@@ -686,12 +685,13 @@ func (e *IndexLookUpExecutor) startIndexWorker(ctx context.Context, workCh chan<
 			SetMemTracker(tracker).
 			SetConnIDAndConnAlias(e.Ctx().GetSessionVars().ConnectionID, e.Ctx().GetSessionVars().SessionAlias)
 
-		if builder.Request.Paging.Enable && builder.Request.Paging.MinPagingSize < uint64(initBatchSize) {
+		worker.batchSize = e.calculateBatchSize(initBatchSize, worker.maxBatchSize)
+		if builder.Request.Paging.Enable && builder.Request.Paging.MinPagingSize < uint64(worker.batchSize) {
 			// when paging enabled and Paging.MinPagingSize less than initBatchSize, change Paging.MinPagingSize to
 			// initBatchSize to avoid redundant paging RPC, see more detail in https://github.com/pingcap/tidb/issues/53827
-			builder.Request.Paging.MinPagingSize = uint64(initBatchSize)
-			if builder.Request.Paging.MaxPagingSize < uint64(initBatchSize) {
-				builder.Request.Paging.MaxPagingSize = uint64(initBatchSize)
+			builder.Request.Paging.MinPagingSize = uint64(worker.batchSize)
+			if builder.Request.Paging.MaxPagingSize < uint64(worker.batchSize) {
+				builder.Request.Paging.MaxPagingSize = uint64(worker.batchSize)
 			}
 		}
 		results := make([]distsql.SelectResult, 0, len(kvRanges))
@@ -724,7 +724,6 @@ func (e *IndexLookUpExecutor) startIndexWorker(ctx context.Context, workCh chan<
 			}
 			results = append(results, result)
 		}
-		worker.batchSize = min(initBatchSize, worker.maxBatchSize)
 		if len(results) > 1 && len(e.byItems) != 0 {
 			// e.Schema() not the output schema for indexReader, and we put byItems related column at first in `buildIndexReq`, so use nil here.
 			ssr := distsql.NewSortedSelectResults(e.Ctx().GetExprCtx().GetEvalCtx(), results, nil, e.byItems, e.memTracker)
@@ -746,6 +745,35 @@ func (e *IndexLookUpExecutor) startIndexWorker(ctx context.Context, workCh chan<
 	return nil
 }
 
+// calculateBatchSize calculates a suitable initial batch size.
+func (e *IndexLookUpExecutor) calculateBatchSize(initBatchSize, maxBatchSize int) int {
+	var estRows int
+	if len(e.idxPlans) > 0 {
+		estRows = int(e.idxPlans[0].StatsCount())
+	}
+	return CalculateBatchSize(e.indexPaging, estRows, initBatchSize, maxBatchSize)
+}
+
+// CalculateBatchSize calculates a suitable initial batch size. It exports for testing.
+func CalculateBatchSize(indexPaging bool, estRows, initBatchSize, maxBatchSize int) int {
+	batchSize := min(initBatchSize, maxBatchSize)
+	if indexPaging {
+		// If indexPaging is true means this query has limit, so use initBatchSize to avoid scan some unnecessary data.
+		return batchSize
+	}
+	if estRows >= maxBatchSize {
+		return maxBatchSize
+	}
+	for batchSize < estRows {
+		// If batchSize less than estRows, increase batch size to avoid unnecessary rpc.
+		batchSize = batchSize * 2
+		if batchSize >= maxBatchSize {
+			return maxBatchSize
+		}
+	}
+	return batchSize
+}
+
 // startTableWorker launches some background goroutines which pick tasks from workCh and execute the task.
 func (e *IndexLookUpExecutor) startTableWorker(ctx context.Context, workCh <-chan *lookupTableTask) {
 	lookupConcurrencyLimit := e.Ctx().GetSessionVars().IndexLookupConcurrency()