Oliver Jowett <[EMAIL PROTECTED]> writes:
I guess that ExprState does not live long enough to be useful.
Actually the opposite: it lasts too long, namely the entire execution of a query. I don't think there's any convenient way to reset it on the timescale appropriate for STABLE values (ie, once per scan, as opposed to once per query).
I think you misunderstand what I was suggesting. Given your earlier clarification of what STABLE means, it isn't correct to mark expressions involving a STABLE function as constant-at-execution-time, so those results would not be cached. But there are still other expression trees that would benefit, e.g. those involving an IMMUTABLE function with parameterized arguments.
How about introducing a function modifier that provides stronger guarantees than STABLE, along the lines of "immutable during execution of a single SQL statement"?
Why?
It's not directly useful currently, as there's no expression caching going on. If there was expression caching, the stronger guarantees would allow you to cache a wider range of expressions.
I suspect that if we did have two flavors of STABLE, we'd just have a lot of people getting it wrong :-(. A big advantage of the current definition is exactly that it is pretty weak...
It seems quite hard to build a STABLE function that doesn't also satisfy the stronger requirements. I can't think of how you'd do it as a SQL function at all, off the top of my head. What sort of function were you thinking of that is STABLE-safe but doesn't satisfy the stronger requirements?
-O
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