Re: index prefetching

On 1/22/24 07:35, Konstantin Knizhnik wrote:
> 
> On 22/01/2024 1:47 am, Tomas Vondra wrote:
>> h, right. Well, you're right in this case we perhaps could set just one
>> of those flags, but the "purpose" of the two places is quite different.
>>
>> The "prefetch" flag is fully controlled by the prefetcher, and it's up
>> to it to change it (e.g. I can easily imagine some new logic touching
>> setting it to "false" for some reason).
>>
>> The "data" flag is fully controlled by the custom callbacks, so whatever
>> the callback stores, will be there.
>>
>> I don't think it's worth simplifying this. In particular, I don't think
>> the callback can assume it can rely on the "prefetch" flag.
>>
> Why not to add "all_visible" flag to IndexPrefetchEntry ? If will not
> cause any extra space overhead (because of alignment), but allows to
> avoid dynamic memory allocation (not sure if it is critical, but nice to
> avoid if possible).
> 

Because it's specific to index-only scans, while IndexPrefetchEntry is a
generic thing, for all places.

However:

(1) Melanie actually presented a very different way to implement this,
relying on the StreamingRead API. So chances are this struct won't
actually be used.

(2) After going through Melanie's patch, I realized this is actually
broken. The IOS case needs to keep more stuff, not just the all-visible
flag, but also the index tuple. Otherwise it'll just operate on the last
tuple read from the index, which happens to be in xs_ituple. Attached is
a patch with a trivial fix.


regards

-- 
Tomas Vondra
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