PHOENIX-4315 function Greatest/Least

[yanxinyi]

Currently, COALESCE supports only 2 arguments.
For instance, the GREATEST(expr1, expr2) is supported but GREATEST(expr1, expr2, expr3, ...) is not.
Only implemented GREATEST for now, but if the list of arguments uses cases are needed from the open source community, We can do the optimization later.
If everyone is ok with this approach, I will create another patch for Least feature.
:)

[dbwong]

Why are we limiting/testing the types for Greatest or is this JIRA specifically for that subset of types?
See other implementations: https://www.postgresql.org/docs/8.1/functions-conditional.html https://www.techonthenet.com/oracle/functions/greatest.php which handle multiple data types outside of the ones allowed.

There are some concerns for handling data types that are more precise than a java double (Standard SQL decimal type). As well as non-double based comparisons.

Second outside of end to end tests which use the mini cluster likley we can also make a smaller unit test for GreatestFunction