first_last_agg -------------- A simple extension providing two aggregate functions, `last` and `first` aggregate functions, operating on any element type and returning the last or the first value of the group. Since by default the ordering inside groups created by a `GROUP BY` expression is not defined, it is advisable to use an `ORDER BY` clause inside the aggregate expression (see the [aggregate function syntax](http://www.postgresql.org/docs/current/static/sql-expressions.html#SYNTAX-AGGREGATES)). This feature has been added in PostgreSQL 9.0. This extension is intended to replace the corresponding [first and last](http://wiki.postgresql.org/wiki/First/last_%28aggregate%29) aggregates from the PostgreSQL wiki.