Added lubridate specifics for Oracle

[rlh1994]

I've supplied the basic changes needed to fully support oracle with the lubridate functions currently supported across all languages.

Main differences are:

• Forced cast to timestamp for dates to get hour/minute/second - this shouldn't introduce much if any overhead to queries where it already is a timestamp.
• Today is based on sysdate as CURRENT_DATE has a time in oracle (note this will be the current date of the system the database is running on, not the date of the NLS settings of the database, this can be changed if it is different to the other SQL variants)
• I've implemented as.Date via an explicit call to Date rather than via a cast - there's 2 reasons for this. The first is that the database optimiser can't identify casts as dates at run time so it might not e.g. identify it should filter on a partition before a join, rather than after. Secondly, the cast to date is NLS setting dependant in Oracle databases; at least in my experience most users will not actually be aware of what this is so the conversion will fail. The method implemented will work for YYYY-MM-DD formats and as as.Date tries YYYY-MM-DD by default this is consistent with the base R method. I do still have the code for the full(ish) version of as.Date/as_date that includes the format argument specific to an Oracle translation however did not include it as the base backend does not support this argument.
• I've also included support for yday and wday as they were listed explicitly in the backend so assumed they would be supported soon and Oracle has a very easy to_char function to extract them.

[hadley]

This doesn't look like valid syntax. Can you please double check?

[rlh1994]

If you meant the R syntax you're right, not sure why I did it that way, it'll be corrected in my commit.
The SQL syntax is the ANSI SQL string literal. At least in Oracle SQL (I can't speak for others) using cast is dependant on the NLS settings, which means unless you know this and format your date string in advance then you can't be certain it will work. the ANSI uses the YYYY-MM-DD format (which matches the default lubridate format string, at least in my localisation?). I thought this would be the safest way to catch most dates short of adding in and translating the format options?
The last script of the introduction section here shows this example https://oracle-base.com/articles/misc/oracle-dates-timestamps-and-intervals

[hadley]

If you're using build_sql() you don't need !!

[rlh1994]

Wasn't aware thanks, corrected in next commit