DOCS-489 aggregation operators and indexes

Author: kay-kim

source/applications/aggregation.txt

@@ -174,20 +174,44 @@ Document size <limit-bson-document-size>` limit, which is currently 16 megabytes
 Optimizing Performance
 ----------------------
 
-Early Filtering
-~~~~~~~~~~~~~~~
-
 Because you will always call :method:`aggregate` on a
 :term:`collection` object, which logically inserts the *entire* collection into
 the aggregation pipeline, you may want to optimize the operation
 by avoiding scanning the entire collection whenever possible.
 
+Aggregation Operators and Indexes
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Depending on the order in which they appear in the pipeline,
+aggregation operators can take advantage of indexes.
+
+The following aggregation operators:
+
+- :agg:pipeline:`$match`
+- :agg:pipeline:`$sort`
+- :agg:pipeline:`$limit` 
+- :agg:pipeline:`$skip` 
+
+can take advantage of an index when placed at the **beginning** of the pipleline or
+placed **before** the following aggregation operators:
+
+- :agg:pipeline:`$project`
+- :agg:pipeline:`$unwind`
+- :agg:pipeline:`$group`.
+
+Early Filtering
+~~~~~~~~~~~~~~~
+
 If your aggregation operation requires only a subset of the data in a
 collection, use the :agg:pipeline:`$match` operator to restrict which items go
 in to the top of the pipeline, as in a query. When placed early in a
 pipeline, these :agg:pipeline:`$match` operations use suitable indexes
 to scan only the matching documents in a collection.
 
+Placing :agg:pipeline:`$match` then :agg:pipeline:`$sort` at the very
+start of the pipeline would be logically equivalent to a single query
+and a sort, and be able to use an index.
+
 .. OMMITED: this feature is pending SERVER-4506. Other optimizations
 .. are pending SERVER-4507 SERVER-4644 SERVER-4656 SERVER-4816
 ..

source/reference/aggregation/sort.txt

@@ -41,6 +41,16 @@ $sort
 
       .. TODO mention the importance of order preserving objects
 
+      - :agg:pipeline:`$skip` 
+
+   :pipeline:`$sort` operator can take advantage of an index when
+   placed at the **beginning** of the pipleline or placed **before**
+   the following aggregation operators:
+
+   - :agg:pipeline:`$project`
+   - :agg:pipeline:`$unwind`
+   - :agg:pipeline:`$group`.
+
    .. warning:: Unless the :pipeline:`$sort` operator can use an index,
       in the current release, the sort must fit within memory. This
       may cause problems when sorting large numbers of documents.