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DOCS-10517: Update unique key for sharded cluster #3197

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DOCS-10517: Update unique key for sharded cluster #3197

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12 changes: 8 additions & 4 deletions source/core/sharding-shard-key.txt
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Expand Up @@ -91,16 +91,20 @@ recreating an index on just the shard key.
Unique Indexes
~~~~~~~~~~~~~~

For a sharded collection, only the ``_id`` field index and the index on
the shard key or a :term:`compound index` where the shard key is a
:ref:`prefix <compound-index-prefix>` can be :doc:`unique
</core/index-unique>`:
For a sharded collection, only the index on the shard key or a :term:`compound
index` where the shard key is a :ref:`prefix <compound-index-prefix>` can be
:doc:`unique </core/index-unique>`:

- You cannot shard a collection that has unique indexes on other fields.

- You cannot create unique indexes on other fields for a sharded
collection.

.. important::

The ``_id`` field index can only be :doc:`unique </core/index-unique>`
across the sharded cluster if it is the shard key.

Through the use of the unique index on the shard key, MongoDB *can*
enforce uniqueness on the shard key values. MongoDB enforces uniqueness
on the *entire* key combination, and not individual components of the
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