On Tue, 2 Apr 2002, Peter Eisentraut wrote: > 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.
SQL99 suggests that there are only two types of user defined routines: deterministic and 'possibly non-deterministic'. However, in section 11.49 it defines <deterministic characteristic> ::= DETERMINISTIC | NOT DETERMINISTIC So the real problem is how to qualify this. TRANSACTIONAL DETERMINISTIC or NOT DETERMINISTIC CACHEABLE are the only ways that come to mind. I'll admit that I don't like either. > > > 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 > > Gavin ---------------------------(end of broadcast)--------------------------- TIP 6: Have you searched our list archives? http://archives.postgresql.org
