Re: the big picture for index-only scans

>> The temptation is high to estimate the cost of an "index_scan(only) +
>> ordered(by ctid) table pages fetch if heap required". (this is what I
>> understood from heikki suggestion 3-4. and it makes sense). It may be
>> easier to implement both at once but I didn't find the branch in the
>> Heikki's git repos. (probably removed since the long time)
>
> I was thinking about this as well, at least if I understand you

yes.

> correctly.  That would be similar to a bitmap index scan, and I think
> it would be a great thing to have, not only because it would allow us
> to get the advantages of index-only scans in situations that are
> well-suited to our current bitmap scans, but also because it could be
> batched.  You could allocate a buffer of work_mem bytes and fill it up
> with TIDs; then, when it's full, you sort the buffer and start doing
> the necessary heap fetches in physical order.  If you still need more
> rows, you can clear the buffer and go around for another pass.

Issue remaining here is that we don't have 'safe' Indexonly_scan, just
indexscan with probability on the 'only'.

--
Cédric Villemain               2ndQuadrant
http://2ndQuadrant.fr/     PostgreSQL : Expertise, Formation et Support