Tom Lane writes:
> Since I'm about to have to edit pg_proc.h to add a namespace column,
> I thought this would be a good time to revise the current proiscachable
> column into the three-way cachability distinction we've discussed
> before. But I need some names for the values, and I'm not satisfied
> with the ideas I've had so far.
Well, for one thing, we might want to change the name to the correct
spelling "cacheable".
> 1. Strictly cachable (a/k/a constant-foldable): given fixed input
> values, the same result value will always be produced, for ever and
> ever, amen. Examples: addition operator, sin(x). Given a call
> of such a function with all-constant input values, the system is
> entitled to fold the function call to a constant on sight.
deterministic
(That's how SQL99 calls it.)
> 2. Cachable within a single command: given fixed input values, the
> result will not change if the function were to be repeatedly evaluated
> within a single SQL command; but the result could change over time.
> Examples: now(); datetime-related operations that depend on the current
> timezone (or other SET-able variables); any function that looks in
> database tables to determine its result.
"cacheable" seems OK for this.
> 3. Totally non-cachable: result may change from one call to the next,
> even within a single SQL command. Examples: nextval(), random(),
> timeofday(). (Yes, timeofday() and now() are in different categories.
> See http://www.ca.postgresql.org/users-lounge/docs/7.2/postgres/functions-datetime.html#FUNCTIONS-DATETIME-CURRENT)
not deterministic, not cacheable
--
Peter Eisentraut peter_e@gmx.net
