Re: Index-only-scans, indexam API changes

Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> writes:
> Tom Lane wrote:
>> What are you going to do for index types that don't store the original
>> data (e.g. hash)?

> They will obviously not be able to regurgitate index tuples. I have not
> yet decided how that's going to be signaled.

Well, I think that's a pretty critical part of the API change.

> (This indexam API patch isn't bothered with that yet. It just splits
> index_gettuple() into two)

One thought here is that an AM call isn't really free, and doing two of
them instead of one mightn't be such a good idea. I would suggest
either having a separate AM entry point to get both bits of data
("amgettupledata"?) or adding an optional parameter to amgettuple.
A small attraction of the alternate entry point is that then you can
flag the "unsupported" case by putting a zero in that pgam column, as
indeed we already do for amgettuple; so you don't need an additional
bool column.

[ thinks a bit ... ] At least for GIST, it is possible that whether
data can be regurgitated will vary depending on the selected opclass.
Some opclasses use the STORAGE modifier and some don't. I am not sure
how hard we want to work to support flexibility there. Would it be
sufficient to hard-code the check as "pgam says the AM can do it,
and the opclass does not have a STORAGE property"? Or do we need
additional intelligence about GIST opclasses?

regards, tom lane