$limit (aggregation)
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Definition
Behavior
Using $limit with Sorted Results
If using the $limit stage with any of:
the
$sortaggregation stage,the
sort()method, orthe
sortfield to thefindAndModifycommand or thefindAndModify()shell method,
be sure to include at least one field in your sort that contains
unique values, before passing results to the $limit stage.
Sorting on fields that contain duplicate values may return an inconsistent sort order for those duplicate fields over multiple executions, especially when the collection is actively receiving writes.
The easiest way to guarantee sort consistency is to include the
_id field in your sort query.
See the following for more information on each:
Example
Consider the following example:
db.article.aggregate([ { $limit : 5 } ]);
This operation returns only the first 5 documents passed to it
by the pipeline. $limit has no effect on the content
of the documents it passes.
Note
When a $sort precedes a $limit and there are no
intervening stages that modify the number of documents, the optimizer can
coalesce the $limit into the $sort. This allows
the $sort operation to only
maintain the top n results as it progresses, where n is the
specified limit, and ensures that MongoDB only needs to store n items in memory.
This optimization still applies when allowDiskUse is true and
the n items exceed the aggregation memory limit.