Support a more fine-grained datediff function in Impala (once it exists)

Impala does not yet support a proper date difference (e.g. milliseconds between or nanoseconds between). While nanoseconds-between can yield int64 overflows on historical dates (pandas supports a little under 500 years range due to using nanosecond resolution), this is still decently useful.

Note that Impala’s datediff function is the only real “timedelta” function, but it only computes days between, which isn’t that useful in general.

[mariusvniekerk]

I would test against a current install of impala. Best to verify what the current behavior is.

A less precise version should be possible by casting both timestamps to int64 and then compute the difference. This will return a delta in seconds if memory serves.

[cpcloud]

This is still in units of days as of 2021: https://impala.apache.org/docs/build/html/topics/impala_datetime_functions.html#datetime_functions__datediff