Re: gincostestimate and hypothetical indexes

Julien Rouhaud <julien(dot)rouhaud(at)dalibo(dot)com> writes:
> I figured out that it's not possible to use a hypothetical gin index, as
> the gincostestimate function try to retrieve some statistical data from
> the index meta page.

Good point.

> Attached patch fixes this. I believe this should be back-patched as was
> a2095f7fb5a57ea1794f25d029756d9a140fd429.

I don't much care for this patch though. The core problem is that just
returning all zeroes seems quite useless: it will probably result in silly
cost estimates. The comment in the patch claiming that this would be the
situation in a never-vacuumed index is wrong, because ginbuild() updates
those stats too. But I'm not sure exactly what to do instead :-(.

Ideally we'd put it on the head of the hypothetical-index plugin to invent
some numbers, but I dunno if we want to create such an API or not ... and
we certainly couldn't back-patch such a change.

Maybe we could do something along the lines of pretending that 90% of the
index size given by the plugin is entry pages? Don't know what a good
ratio would be exactly, but we could probably come up with one with a bit
of testing.

regards, tom lane