row: release memory more eagerly in txnKVFetcher

Previously, we could leave some dangling references to batch responses
around in the txnKVFetcher when we were fetching more than one batch at
a time. This would cause a delay in reclamation of memory for the
lifetime of a given query.

Release note (bug fix): use less memory in some queries, primarily
lookup joins.

pkg/sql/row/kv_batch_fetcher.go

@@ -525,26 +525,33 @@ func (f *txnKVFetcher) nextBatch(
 ) (ok bool, kvs []roachpb.KeyValue, batchResponse []byte, origSpan roachpb.Span, err error) {
  if len(f.remainingBatches) > 0 {
  batch := f.remainingBatches[0]
+ f.remainingBatches[0] = nil
  f.remainingBatches = f.remainingBatches[1:]
  return true, nil, batch, f.origSpan, nil
  }
  for len(f.responses) > 0 {
  reply := f.responses[0].GetInner()
+ f.responses[0] = roachpb.ResponseUnion{}
  f.responses = f.responses[1:]
  origSpan := f.requestSpans[0]
+ f.requestSpans[0] = roachpb.Span{}
  f.requestSpans = f.requestSpans[1:]
  var batchResp []byte
  switch t := reply.(type) {
  case *roachpb.ScanResponse:
  if len(t.BatchResponses) > 0 {
  batchResp = t.BatchResponses[0]
  f.remainingBatches = t.BatchResponses[1:]
+ t.BatchResponses[0] = nil
+ t.BatchResponses = nil
  }
  return true, t.Rows, batchResp, origSpan, nil
  case *roachpb.ReverseScanResponse:
  if len(t.BatchResponses) > 0 {
  batchResp = t.BatchResponses[0]
  f.remainingBatches = t.BatchResponses[1:]
+ t.BatchResponses[0] = nil
+ t.BatchResponses = nil
  }
  return true, t.Rows, batchResp, origSpan, nil
  case *roachpb.GetResponse:
@@ -571,5 +578,7 @@ func (f *txnKVFetcher) nextBatch(
 
 // close releases the resources of this txnKVFetcher.
 func (f *txnKVFetcher) close(ctx context.Context) {
+ f.responses = nil
+ f.remainingBatches = nil
  f.acc.Close(ctx)
 }