On 20/2/2024 11:03, jian he wrote:
>> Neither the code comments nor the commit message really explain the
>> design idea here. That's unfortunate, principally because it makes
>> review difficult.
>>
>> I'm very skeptical about the idea of using JumbleExpr for any part of
>> this. It seems fairly expensive, and it might produce false matches.
>> If expensive is OK, then why not just use equal()? If it's not, then
>> this probably isn't really OK either. But in any case there should be
>> comments explaining why this strategy was chosen.
>
> The above message
> (https://postgr.es/m/CA%2BTgmoZCgP6FrBQEusn4yaWm02XU8OPeoEMk91q7PRBgwaAkFw%40mail.gmail.com)
> seems still not answered.
> How can we evaluate whether JumbleExpr is expensive or not?
> I used this naive script to test, but didn't find a big difference
> when enable_or_transformation is ON or OFF.
First, I am open to discussion here. But IMO, equal() operation is quite
expensive by itself. We should use the hash table approach to avoid
quadratic behaviour when looking for similar clauses in the 'OR' list.
Moreover, we use equal() in many places: selectivity estimations,
proving of fitting the index, predtest, etc. So, by reducing the clause
list, we eliminate many calls of the equal() routine, too.
> `leftop operator rightop`
> the operator can also be volatile.
> Do we need to check (op_volatile(opno) == PROVOLATILE_VOLATILE) within
> transformBoolExprOr?
As usual, could you provide a test case to discuss it more objectively?
--
regards,
Andrei Lepikhov
Postgres Professional