Re: WIP: generalized index constraints

Right now this patch does not support GIN because GIN doesn't support
amgettuple.

It could be made to support GIN by doing a bitmap index scan, manually
fetching the next tuple (or, if it's lossy, the next one on the page),
checking it against the snapshot, and then rechecking it to make sure it
still matches.

The API I'm looking for is essentially the same as index_getnext(),
which makes the most sense for finding constraint violations.

Is it possible to re-add amgettuple to GIN, and just set the cost high
so it's not chosen by the planner? Or is there some reason this is
fundamentally a bad idea (or won't work at all)?

I know we removed it in 8.4:
http://archives.postgresql.org/pgsql-hackers/2009-02/msg01123.php
http://archives.postgresql.org/pgsql-hackers/2009-02/msg00532.php

but the reasoning seemed mostly because:
1. GIN could not support amgettuple efficiently (lots of rechecking)
2. Normal index scans did not fit a common use case for GIN, anyway

However, if my feature needs to perform this check anyway (to support
GIN, that is), it seems like it could be re-added. There was also some
resistance to removing it in the first place (e.g. for anti-joins), so
perhaps it can be made to be efficient again during the 8.5 cycle.

Regards,
Jeff Davis