Re: pgsql_fdw, FDW for PostgreSQL server

Shigeru Hanada <shigeru(dot)hanada(at)gmail(dot)com> writes:
> (2011/12/12 22:59), Robert Haas wrote:
>> ... I feel like we might need a system here that
>> allows for more explicit user control about what to push down vs. not,
>> rather than assuming we'll be able to figure it out behind the scenes.

> Agreed. How about to add a per-column boolean FDW option, say
> "pushdown", to pgsql_fdw? Users can tell pgsql_fdw that the column can
> be pushed down safely by setting this option to true.

[ itch... ] That doesn't seem like the right level of granularity.
ISTM the problem is with whether specific operators have the same
meaning at the far end as they do locally. If you try to attach the
flag to columns, you have to promise that *every* operator on that
column means what it does locally, which is likely to not be the
case ever if you look hard enough. Plus, having to set the flag on
each individual column of the same datatype seems pretty tedious.

I don't have a better idea to offer at the moment though. Trying
to attach such a property to operators seems impossibly messy too.
If it weren't for the collations issue, I might think that labeling
datatypes as being compatible would be a workable approximation.

regards, tom lane