Re: pg9.6 segfault using simple query (related to use fk for join estimates)

I think we can further limit the impact by ignoring foreign keys on a single column, because the primary goal of the patch is improving estimates with multi-column FKs (and matching joins). I'd argue that 99% of the FKs in practice is single-column ones, but sure - if you have a database with many multi-column FKs this won't help.

I think it's also important to point out that whatever solution we choose, it should probably allow us to relax some of the restrictions in the future. For example, with a FK on 3 columns, the current patch only handles a "full match" joins, with conditions on all three columns. But it may be possible to also improve estimates on just two columns - the current patch does not do that, as that needs definitely further more thought and discussion.