Single document updates usually need to specify an exact match on the shard key to be allowed, so that mongos knows it can send it to a single shard and be guaranteed to only update a single document. If, however, your shard key doesn't include _id, we allow you to do a single update that matches on _id but not the shard key. This will result in sending the write to all shards, with the expectation that only one shard will contain a document with that _id. Since we don't enforce uniqueness of _id in a sharded cluster that's not sharded on _id, however, that assumption is incorrect and a single update could in fact wind up updating multiple unrelated documents.
- is duplicated by
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SERVER-31943 Two documents can share the same documentKey when using a non-simple collation.
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- Closed
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- is related to
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- Closed
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- related to
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SERVER-34971 Improve mongoS targeting of replacement-style updates for collections whose shard key includes _id
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- Closed
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