DOCS-14794 exists update (mongodb#6100) (mongodb#6104)

Co-authored-by: jason-price-mongodb <[email protected]>

Co-authored-by: jason-price-mongodb <[email protected]>

Author: jason-price-mongodb

source/reference/operator/query/exists.txt

@@ -103,3 +103,78 @@ The results consist of those documents that do not contain the field
    { a: 2, c: 5 }
    { a: 4 }
    { c: 6 }
+
+Use a Sparse Index to Improve ``$exists`` Performance 
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The following scenario is not optimal because all of the collection's
+documents are examined:
+
+- You use a query to retrieve or count documents, and
+- use ``field: { $exists: true }``, and
+- the ``field`` has a non-:ref:`sparse index <index-type-sparse>` or
+  does not have an index.
+
+To improve performance, create a :ref:`sparse index <index-type-sparse>`
+on the ``field`` as shown in the following scenario:
+
+#. Create a ``stockSales`` collection:
+
+   .. code-block:: javascript
+
+      db.stockSales.insertMany( [
+         { _id: 0, symbol: "ABC", auditDate: new Date( "2021-05-18T16:12:23Z" ) },
+         { _id: 1, symbol: "ABC", auditDate: new Date( "2021-04-21T11:34:45Z" ) },
+         { _id: 2, symbol: "DEF", auditDate: new Date( "2021-02-24T15:11:32Z" ) },
+         { _id: 3, symbol: "DEF", auditDate: null },
+         { _id: 4, symbol: "DEF", auditDate: new Date( "2021-07-13T18:32:54Z" ) },
+         { _id: 5, symbol: "XYZ" }
+      ] )
+
+   The document with an ``_id`` of:
+
+   - ``3`` has a null ``auditDate`` value.
+   - ``5`` is missing the ``auditDate`` value.
+
+#. Create a :ref:`sparse index <index-type-sparse>` on the
+   ``auditDate`` field:
+
+   .. code-block:: javascript
+
+      db.getCollection( "stockSales" ).createIndex( 
+         { auditDate: 1 },
+         { name: "auditDateSparseIndex", sparse: true }
+      )
+
+#. The following example counts the documents where the ``auditDate``
+   field has a value (including null) and uses the :ref:`sparse index
+   <index-type-sparse>`:
+
+   .. code-block:: javascript
+
+      db.stockSales.countDocuments( { auditDate: { $exists: true } } )
+
+   The example returns 5. The document that is missing the ``auditDate``
+   value is not counted.
+
+.. tip::
+
+   If you only need documents where the ``field`` has a non-null value,
+   you:
+   
+   - Can use ``$ne: null`` instead of ``$exists: true``.
+   - Do not need a :ref:`sparse index <index-type-sparse>` on the
+     ``field``.
+   
+   For example, using the ``stockSales`` collection:
+
+   .. code-block:: javascript
+
+      db.stockSales.countDocuments( { auditDate: { $ne: null } } )
+
+   The example returns 4. Documents that are missing the ``auditDate``
+   value or have a null ``auditDate`` value are not counted.
+
+.. seealso::
+
+   :doc:`/tutorial/query-for-null-fields`