Transform datetime and timespan into timestamp and interval.
Deprecate datetime and timespan, though translate to new types in gram.y.
Transform all datetime and timespan catalog entries into new types.
Make "INTERVAL" reserved word allowed as a column identifier in gram.y.
Remove dt.h, dt.c files, and retarget datetime.h, datetime.c as utility
routines for all date/time types.
date.{h,c} now deals with date, time types.
timestamp.{h,c} now deals with timestamp, interval types.
nabstime.{h,c} now deals with abstime, reltime, tinterval types.
Make NUMERIC a known native type for purposes of type coersion. Not tested.
SQL cast constructs can be performed during expression transformation
instead of during parsing. This allows constructs like x::numeric(9,2)
and x::int2::float8 to behave as one would expect.
Change ordering of IS_BUILTIN_TYPE() macro to optimize lookup.
Make CASH type _not_ equivalent to INT4.
CASH is passed by reference rather than passed by value.
