WRITING-904 updates to compound indexes per audit

source/core/index-compound.txt

@@ -110,39 +110,41 @@ following:
 Prefixes
 --------
 
-Compound indexes support queries on any prefix of the index
-fields. Index prefixes are the beginning subset of indexed fields. For
-example, given the index ``{ a: 1, b: 1, c: 1 }``, both ``{ a: 1 }``
-and ``{ a: 1, b: 1 }`` are prefixes of the index.
+Compound indexes support queries on any prefix of the index fields.
+Index prefixes are the beginning subsets of indexed fields. For example,
+``{ a: 1 }`` and ``{ a: 1, b: 1 }`` are both prefixes of the index
+``{ a: 1, b: 1, c: 1 }``.
 
-If you have a collection that has a compound index on ``{ a: 1, b:
-1 }``, as well as an index that consists of the prefix of that index,
-i.e. ``{ a: 1 }``, assuming none of the index has a sparse or unique
-constraints, then you can drop the ``{ a: 1 }`` index. MongoDB will be
-able to use the compound index in all of situations that it would have
-used the ``{ a: 1 }`` index.
+If an index is an exact prefix of a second index, and if neither index has
+a sparse or unique constraint, then you can remove the first index. For
+example, if you have an index on ``{ a: 1 }`` and also have a compound
+index on ``{ a: 1, b: 1 }``, then you can remove the ``{ a: 1 }`` index.
+MongoDB will use the ``{ a: 1, b: 1 }`` index in all of the situations
+that it would have used the ``{ a: 1 }`` index.
 
-For example, given the following index:
+Given the following compound index:
 
 .. code-block:: javascript
 
    { "item": 1, "location": 1, "stock": 1 }
 
-MongoDB **can** use this index to support queries that include:
+MongoDB can use the index for queries on the following fields, which are
+prefixes of the index:
 
-- the ``item`` field,
-- the ``item`` field *and* the ``location`` field,
-- the ``item`` field *and* the ``location`` field *and* the
-  ``stock`` field, or
-- only the ``item`` *and* ``stock`` fields; however, this index
-  would be less efficient than an index on only ``item`` and
-  ``stock``.
+- ``item``
+- ``item``, ``location``
 
-MongoDB **cannot** use this index to support queries that include:
+MongoDB **cannot** use the index for queries on the following fields, none
+of which are prefixes of the index:
 
-- only the ``location`` field,
-- only the ``stock`` field, or
-- only the ``location`` *and* ``stock`` fields.
+- only on the ``location`` field
+- only on the ``stock`` field
+- only on the ``location`` and ``stock`` fields
+
+MongoDB **can** use the index to support a query on ``item`` and
+``stock``, even though they are not a prefix. However, the index would not
+be as efficient in supporting the query as would be an index on only
+``item`` and ``stock``.
 
 Index Intersection
 ------------------