Moin,
On 2019-10-20 13:30, Andreas Karlsson wrote:
On 10/1/19 9:38 AM, Andres Freund wrote:
We spend a surprising amount of time during expression evaluation to
reevaluate whether input to a strict function (or similar) is not
null, even though the value either comes from a strict function, or a
column declared not null.
Now you can rightfully say that a strict function still can return
NULL, even when called with non-NULL input. But practically that's
quite rare. Most of the common byvalue type operators are strict, and
approximately none of those return NULL when actually called.
That makes me wonder if it's worthwhile to invent a function property
declaring strict strictness or such. It'd allow for some quite
noticable improvements for e.g. queries aggregating a lot of rows, we
spend a fair time checking whether the transition value has "turned"
not null. I'm about to submit a patch making that less expensive, but
it's still expensive.
I can also imagine that being able to propagate NOT NULL further up
the parse-analysis tree could be beneficial for planning, but I've not
looked at it in any detail.
Agreed, this sounds like something useful to do since virtually all
strict functions cannot return NULL, especially the ones which are
used in tight loops. The main design issue seems to be to think up a
name for this new level of strictness which is not too confusing for
end users.
STRICT NONULL? That way you could do
CREATE FUNCTION f1 ... STRICT;
CREATE FUNCTION f2 ... STRICT NONULL;
CREATE FUNCTION f3 ... NONULL;
and the last wold throw "not implementet yet"? "NEVER RETURNS NULL"
would also ryme with the existing "RETURNS NULL ON NULL INPUT", but I
find the verbosity too high.
Best regards,
Tels
--
Best regards,
Tels
