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Return zero-bit row (with Index/Field) instead of nil in executeDistinctShardSet

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Travis 2021-01-26 15:41:47 -06:00
parent 4f47862b48
commit 315cad679d
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1 changed files with 12 additions and 2 deletions
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14
executor.go
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@ -151,7 +151,6 @@ func (e *executor) Close() error {
// Execute executes a PQL query.
func (e *executor) Execute(ctx context.Context, index string, q *pql.Query, shards []uint64, opt *execOptions) (QueryResponse, error) {
span, ctx := tracing.StartSpanFromContext(ctx, "Executor.Execute")
span.LogKV("pql", q.String())
defer span.Finish()
@ -1513,7 +1512,18 @@ func executeDistinctShardSet(ctx context.Context, qcx *Qcx, idx *Index, fieldNam
fragData, _, err := tx.ContainerIterator(index, fieldName, "standard", shard, 0)
switch errors.Cause(err) {
case ViewNotFound, FragmentNotFound:
return nil, nil
// It may seem reasonable to return `nil` here in the case where the
// fragment for this shard does not exist. The problem with doing that
// is that if this operation is being performed on a remote node, then
// this result is going to get serialized as a QueryResponse and sent
// back to the original, non-remote node. When this happens, the
// encodeRow/decodeRow logic replaces `nil` with an empty `Row`. An
// empty Row will cause problems during the union step of the reduce
// phase if it is the "left" side of the union, because then the
// resulting Row after the union will have blank Index and Field values.
// Here, we ensure that we send a non-nil Row with valid Index and Field
// values so that the union step doesn't cause problems.
return &Row{Index: index, Field: fieldName}, nil
case nil:
default:
return nil, errors.Wrap(err, "getting fragment data")