Implement the use of scalar functions inside aggregate functions.
This allows for complex expressions inside aggregations, with or without
GROUBY as well as with or without a HAVING clause. e.g.:
```
SELECT MAX(CASE WHEN a IS NULL then -1 ELSE abs(a * 10) + 1 END) AS max, b
FROM test
GROUP BY b
HAVING MAX(CASE WHEN a IS NULL then -1 ELSE abs(a * 10) + 1 END) > 5
```
Scalar functions are still not allowed for `KURTOSIS` and `SKEWNESS` as
this is currently not implemented on the ElasticSearch side.
Fixes: #29980Fixes: #36865Fixes: #37271
To avoid having to specify each spec by hand (which can miss specs to be
added), the test infrastructure now performs classpath discovery so that
each spec added, is automatically considered.
Relates #40358
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
* provide overriden `hashCode` and toString methods to account for `DISTINCT`
* change the analyzer for scenarios where `COUNT <field_name>` and `COUNT DISTINCT` have different paths
* defined a new `filter` aggregation encapsulating an `exists` query to filter out null or missing values
Enhance error message for the case that the 2nd argument of PERCENTILE
and PERCENTILE_RANK is not a foldable, as it doesn't make sense to have
a dynamic value coming from a field.
Fixes: #36903
* 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
Split function section into multiple chapters
Add String functions
Add (small) section on Conversion/Cast functions
Add missing aggregation functions
Enable documentation testing (was disabled by accident). While at it,
fix failing tests
Improve spec tests to allow multi-line queries (useful for docs)
Add ability to ignore a spec test (name should end with -Ignore)
