!CHAPTER Features and Improvements

The following list shows in detail which features have been added or improved in
ArangoDB 3.1. ArangoDB 3.1 also contains several bugfixes that are not listed
here.

!SECTION Communication Layer

ArangoDB up to 3.0 used [libev](http://software.schmorp.de/pkg/libev.html) for
the communication layer. ArangoDB starting from 3.1 uses
[Boost ASIO](www.boost.org).

A few options have changed concerning communication, please checkout
[Incompatible changes in 3.1](ReleaseNotes/UpgradingChanges31.html).


!SECTION AQL

!SUBSECTION Functions added

The following AQL functions have been added in 3.1:

- *OUTERSECTION(array1, array2, ..., arrayn)*: returns the values that occur
  only once across all arrays specified.

- *DISTANCE(lat1, lon1, lat2, lon2)*: returns the distance between the two
  coordinates specified by *(lat1, lon1)* and *(lat2, lon2)*. The distance is
  calculated using the haversine formula.

- *JSON_STRINGIFY(value)*: returns a JSON string representation of the value.

- *JSON_PARSE(value)*: converts a JSON-encoded string into a regular object


!SUBSECTION Index usage in traversals
 
3.1 allows AQL traversals to use other indexes than just the edge index.
Traversals with filters on edges can now make use of more specific indexes. For
example, the query

    FOR v, e, p IN 2 OUTBOUND @start @@edge 
      FILTER p.edges[0].foo == "bar"
      RETURN [v, e, p]

may use a hash index on `["_from", "foo"]` instead of the edge index on just 
`["_from"]`.


!SUBSECTION Optimizer improvements

Make the AQL query optimizer inject filter condition expressions referred to 
by variables during filter condition aggregation. For example, in the following 
query

    FOR doc IN collection
      LET cond1 = (doc.value == 1)
      LET cond2 = (doc.value == 2)
      FILTER cond1 || cond2
      RETURN { doc, cond1, cond2 }

the optimizer will now inject the conditions for `cond1` and `cond2` into the 
filter condition `cond1 || cond2`, expanding it to `(doc.value == 1) || (doc.value == 2)` 
and making these conditions available for index searching.

Note that the optimizer previously already injected some conditions into other 
conditions, but only if the variable that defined the condition was not used elsewhere. 
For example, the filter condition in the query

    FOR doc IN collection
      LET cond = (doc.value == 1)
      FILTER cond
      RETURN { doc }

already got optimized before because `cond` was only used once in the query and the 
optimizer decided to inject it into the place where it was used.

This only worked for variables that were referred to once in the query. When a variable 
was used multiple times, the condition was not injected as in the following query

    FOR doc IN collection
      LET cond = (doc.value == 1)
      FILTER cond
      RETURN { doc, cond }

3.1 allows using this condition so that the query can use an index on `doc.value` 
(if such index exists).


!SUBSECTION Miscellaneous improvements

The performance of the `[*]` operator was improved for cases in which this operator
did not use any filters, projections and/or offset/limits.


!SECTION Audit Log

Audit logging has been added, see [Auditing](Auditing/index.html).


!SECTION Client tools

Added option `--skip-lines` for arangoimp
This allows skipping the first few lines from the import file in case the CSV or TSV 
import are used and some initial lines should be skipped from the input.