Please add ROUND function to Druid SQL

[vogievetsky]

It is already implemented as part of http://druid.io/docs/latest/misc/math-expr.html
Please expose it in SQL.

In the meantime one can do:

CASE WHEN X - FLOOR(X) < 0.5 THEN FLOOR(X) ELSE CEIL(X) END

[gianm]

NB: It is not fully implemented - SQL ROUND is more involved than what we support in the expr language today. So we would need to extend the expr language a bit. Also, we have only implemented it for doubles, but we should implement it for integers too (if the "length" arg is negative then rounding an integer does make sense, and it should return a long not a double).

This should involved editing "Round" in io.druid.math.expr.Function and adding a io.druid.sql.calcite.expression.builtin.RoundOperatorConversion. The Round expr might want to become an ExprMacro so it can optimize for the likely case that the second parameter is null or a constant.

See also,

• https://docs.oracle.com/cd/B19306_01/server.102/b14200/functions135.htm
• https://docs.microsoft.com/en-us/sql/t-sql/functions/round-transact-sql?view=sql-server-2017
• https://dev.mysql.com/doc/refman/5.7/en/mathematical-functions.html#function_round

[vogievetsky]

TIL that SQL ROUND is more complex than I thought

[vogievetsky]

This is done