diff options
Diffstat (limited to 'src/mongo/db/query/get_executor.h')
| -rw-r--r-- | src/mongo/db/query/get_executor.h | 12 |
1 files changed, 8 insertions, 4 deletions
diff --git a/src/mongo/db/query/get_executor.h b/src/mongo/db/query/get_executor.h index 4eecc309d99..0e97efa0fe0 100644 --- a/src/mongo/db/query/get_executor.h +++ b/src/mongo/db/query/get_executor.h @@ -150,12 +150,12 @@ bool turnIxscanIntoDistinctIxscan(QuerySolution* soln, * or an aggregation pipeline that uses a $group stage with distinct-like semantics. * * Distinct is unique in that it doesn't care about getting all the results; it just wants all - * possible values of a certain field. As such, we can skip lots of data in certain cases (see - * body of method for detail). + * possible values of a certain field. As such, we can skip lots of data in certain cases (see body + * of method for detail). * * A $group stage on a single field behaves similarly to a distinct command. If it has no - * accumulators or only $first accumulators, the $group command only needs to visit one document - * for each distinct value of the grouped-by (_id) field to compute its result. When there is a sort + * accumulators or only $first accumulators, the $group command only needs to visit one document for + * each distinct value of the grouped-by (_id) field to compute its result. When there is a sort * order specified in parsedDistinct->getQuery()->getQueryRequest.getSort(), the DISTINCT_SCAN will * follow that sort order, ensuring that it chooses the correct document from each group to compute * any $first accumulators. @@ -166,6 +166,10 @@ bool turnIxscanIntoDistinctIxscan(QuerySolution* soln, * DISTINCT_SCAN to filter some but not all duplicates (so that de-duplication is still necessary * after query execution), or it may fall back to a regular IXSCAN. * + * Note that this function uses the projection in 'parsedDistinct' to produce a covered query when + * possible, but when a covered query is not possible, the resulting plan may elide the projection + * stage (instead returning entire fetched documents). + * * For example, a distinct query on field 'b' could use a DISTINCT_SCAN over index {a: 1, b: 1}. * This plan will reduce the output set by filtering out documents that are equal on both the 'a' * and 'b' fields, but it could still output documents with equal 'b' values if their 'a' fields are |