"Paul Ramsey" <pramsey@cleverelephant.ca> writes:
> The "optimized" form gets cached and retrieved from a memory context.
> Each time the function is run within a statement it checks the cache,
> and sees if one of its arguments are the same as the last time around.
> If so, it uses the prepared version of that argument. If not, it
> builds a new prepared version and caches that.
> The key here is being able to check the identify of the arguments...
> is this argument A the same as the one we processed last time? One way
> is to do a memcmp. But it seems likely that PgSQL knows exactly
> whether it is running a nested loop, or a literal, and could tell
> somehow that argument A is the same with each call.
Not really. Certainly there's no way that that information would
propagate into function calls.
In the special case where your argument is a literal constant, I think
there is enough information available to detect that that's the case
(look at get_fn_expr_argtype). But if it's not, there's no very good
way to know whether it's the same as last time.
Perhaps it would be worth changing your on-disk storage format to allow
cheaper checking? For instance include a hash value.
regards, tom lane