Currently aggregate functions can operate only directly on fields.
They cannot be used on top of scalar functions as painless scripting
is currently not supported.
FIRST and LAST can be used with one argument and work similarly to MIN
and MAX but they are implemented using a Top Hits aggregation and
therefore can also operate on keyword fields. When a second argument is
provided then they return the first/last value of the first arg when its
values are ordered ascending/descending (respectively) by the values of
the second argument. Currently because of the usage of a Top Hits
aggregation FIRST and LAST cannot be used in the HAVING clause of a
GROUP BY query to filter on the results of the aggregation.
Closes: #35639
* Added Limitations page
* Made the aggregations page follow the common template for functions
* Modified all tables to have the first row's cells content centered
* Polishing in other various sections
Marios Trivyzas
Andrei Stefan