Add section on indexing in user documentation

Summary:
The openCypher chapter of user documentation now includes information on
creating label & property index.

Reviewers: florijan, buda

Reviewed By: florijan

Subscribers: pullbot

Differential Revision: https://phabricator.memgraph.io/D537

docs/user_technical/open-cypher.md

@@ -11,6 +11,7 @@ not yet supported features of the language are listed.
   * [Reading existing Data](#reading-existing-data)
   * [Writing new Data](#writing-new-data)
   * [Reading & Writing](#reading-amp-writing)
+  * [Indexing](#indexing)
   * [Other Features](#other-features)
 
 ### Reading existing Data
@@ -303,6 +304,54 @@ Example. Set a different properties depending on what `MERGE` did.
 For more details, click [this
 link](https://neo4j.com/docs/developer-manual/current/cypher/clauses/merge/).
 
+### Indexing
+
+An index stores additional information on certain types of data, so that
+retrieving said data becomes more efficient. Downsides of indexing are:
+
+  * requiring extra storage for each index and
+  * slowing down writes to the database.
+
+Carefully choosing which data to index can tremendously improve data retrieval
+efficiency, and thus make index downsides negligible.
+
+Memgraph automatically indexes labeled data. This improves queries
+which fetch nodes by label:
+
+    MATCH (n :Label) ... RETURN n
+
+Indexing can also be applied to data with a specific combination of label and
+property. These are not automatically created, instead a user needs to create
+them explicitly. Creation is done using a special
+`CREATE INDEX ON :Label(property)` language construct.
+
+For example, to index nodes which is labeled as `:Person` and has a property
+named `age`:
+
+    CREATE INDEX ON :Person(age)
+
+After the index is created, retrieving those nodes will become more efficient.
+For example, the following query will retrieve all nodes which have an `age`
+property, instead of fetching each `:Person` node and checking whether the
+property exists.
+
+    MATCH (n :Person {age: 42}) RETURN n
+
+Using index based retrieval also works when filtering labels and properties
+with `WHERE`. For example, the same effect as in the previous example can be
+done with:
+
+    MATCH (n) WHERE n:Person AND n.age = 42 RETURN n
+
+Since the filter inside `WHERE` can contain any kind of an expression, the
+expression can be complicated enough so that the index does not get used. We
+are continuously improving the recognition of index usage opportunities from a
+`WHERE` expression. If there is any suspicion that an index may not be used,
+we recommend putting properties and labels inside the `MATCH` pattern.
+
+Currently, once an index is created it cannot be deleted. This feature will be
+implemented very soon. The expected syntax for removing an index will be `DROP
+INDEX ON :Label(property)`.
 
 ### Other Features
 

docs/user_technical/upcoming-features.md

@@ -14,20 +14,13 @@ will be continually working on improving the performance. This includes:
   * memory usage and
   * other improvements.
 
-### Indexing by Label-Property
+### Label-Property Index Usage Improvements
 
-Currently, all nodes in the database are indexed by their labels (if they have
+Currently, indexing combinations of labels and properties can be created, but
-any). This improves the performance of finding labeled data. We plan to extend
+cannot be deleted. We plan to add a new query language construct which will
-indexing to include combinations of labels and property values. These kinds of
+allow deletion of created indices.
-indexes would need to be created on demand, by using a new query language
-construct.
 
-The idea behind label-property indexing is to improve the performance of
+### Improving openCypher Support
-finding data, which contains a specific combination of labels and property
-values. In addition to finding the exact values, with this mechanism, we will
-offer faster retrieving of property values in sorted order.
-
-### Improving openCypher support
 
 Although we have implemented the most common features of the openCypher query
 language, there are other useful features we are still working on.