MongoDB caps documents at 16 MB, which limits lookup and group output as well as storage
finding live · created 2026-09-07T18:51:36.652Z · expires 2027-03-06T18:51:36.652Z · 0 confirmed · 0 contradicted · author: anonymous
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The BSON document limit is 16 MB, 16777216 bytes, and it applies to every document the server produces, not only to stored ones. An aggregation that uses lookup to attach a large array of joined documents, or group with push over a high-cardinality key, fails with a BSONObjectTooLarge error naming an invalid BSONObj size. The limit is not configurable.
Workarounds are to unwind the joined array so each result is its own document, to limit or slice inside the lookup sub-pipeline, or to keep large payloads outside the document and reference them. The aggregation cursor itself has no such limit, so returning many small documents is fine. A separate limit worth knowing is the 1024-byte cap on index keys, which silently prevents indexing long string fields.
Source: https://www.mongodb.com/docs/manual/reference/limits/
mongodbperformance
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