DOCS-10517: unique index and shard keys

Author: Kay Kim

source/core/index-unique.txt

@@ -220,3 +220,46 @@ A partial index with a unique constraint does not prevent the insertion
 of documents that do not meet the unique constraint if the documents do
 not meet the filter criteria. For an example, see
 :ref:`partial-index-with-unique-constraints`.
+
+Sharded Clusters and Unique Indexes
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+You cannot specify a unique constraint on a :ref:`hashed index
+<index-type-hashed>`.
+
+For a ranged sharded collection, only the following indexes can be
+:doc:`unique </core/index-unique>`:
+
+- the index on the shard key
+
+- a :term:`compound index` where the shard key is a :ref:`prefix
+  <compound-index-prefix>`
+
+- the default ``_id`` index; **however**, the ``_id`` index only
+  enforces the uniqueness constraint per shard **if** the ``_id`` field
+  is not the shard key or the prefix of the shard key.
+
+  .. admonition:: Uniqueness and the ``_id`` Index
+     :class: important
+
+     If the ``_id`` field is not the shard key or the prefix of the
+     shard key, ``_id`` index only enforces the uniqueness constraint
+     per shard and **not** across shards.
+
+     For example, consider a sharded collection (with shard key ``{x:
+     1}``) that spans two shards A and B. Because the ``_id`` key is
+     not part of the shard key, the collection could have a document
+     with ``_id`` value ``1`` in shard A and another document with
+     ``_id`` value ``1`` in shard B.
+
+     If the ``_id`` field is not the shard key nor the prefix of the
+     shard key, MongoDB expects applications to enforce the uniqueness
+     of the ``_id`` values across the shards.
+
+The unique index constraints mean that:
+
+- For a to-be-sharded collection, you cannot shard the collection if
+  the collection has other unique indexes.
+
+- For an already-sharded collection, you cannot create unique indexes
+  on other fields.

source/core/sharding-shard-key.txt

@@ -91,6 +91,9 @@ recreating an index on just the shard key.
 Unique Indexes
 ~~~~~~~~~~~~~~
 
+You cannot specify a unique constraint on a :ref:`hashed index
+<index-type-hashed>`.
+
 For a ranged sharded collection, only the following indexes can be
 :doc:`unique </core/index-unique>`:
 
@@ -99,20 +102,27 @@ For a ranged sharded collection, only the following indexes can be
 - a :term:`compound index` where the shard key is a :ref:`prefix
   <compound-index-prefix>`
 
-- the default ``_id`` index
+- the default ``_id`` index; **however**, the ``_id`` index only
+  enforces the uniqueness constraint per shard **if** the ``_id`` field
+  is not the shard key or the prefix of the shard key.
+
+  .. admonition:: Uniqueness and the ``_id`` Index
+     :class: important
 
-  .. note::
-  
-     If the ``_id`` field is not part of the shard key, ``_id`` index
-     only enforces the uniqueness constraint per shard and **not**
-     across shards.
+     If the ``_id`` field is not the shard key or the prefix of the
+     shard key, ``_id`` index only enforces the uniqueness constraint
+     per shard and **not** across shards.
 
      For example, consider a sharded collection (with shard key ``{x:
      1}``) that spans two shards A and B. Because the ``_id`` key is
      not part of the shard key, the collection could have a document
      with ``_id`` value ``1`` in shard A and another document with
      ``_id`` value ``1`` in shard B.
 
+     If the ``_id`` field is not the shard key nor the prefix of the
+     shard key, MongoDB expects applications to enforce the uniqueness
+     of the ``_id`` values across the shards.
+
 The unique index constraints mean that:
 
 - For a to-be-sharded collection, you cannot shard the collection if
@@ -130,9 +140,6 @@ method:
 
 .. include:: /includes/extracts/shard-collection-unique-restriction-method.rst
 
-You cannot specify a unique constraint on a :ref:`hashed index
-<index-type-hashed>`.
-
 .. index:: shard key; cardinality
 
 .. _shard-key-selection-divisible: