On Thu, Apr 14, 2011 at 8:26 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > 1. Accepting non-functional constraint declarations is something we've > been heard to ridicule mysql for. With good reason. > > 2. It probably won't be too long before the planner makes optimization > decisions that assume NOT NULL declarations to be truthful. When that > day comes, I don't want to be seeing an exception for foreign tables in > that logic. > > 3. When we do get around to making it actually work, we will have a > backwards-compatibility problem if prior versions accepted the > declaration but treated it as a no-op.
The planner *already* makes optimization decisions that assume NOT NULL declarations are truthful (see: left join reordering). That's why we have them for foreign tables, and why we eventually will need constraints as well. Of course, we have no guarantee that the data on the foreign side matches those constraints. But we don't have any guarantee that it matches the data type declarations, either. We could insist that every column must be of type text and allow NULLs, but that seems like it would be losing rather a lot of the functionality. The point of a datatype declaration, or a NOT NULL declaration, or any other such thing with respect to foreign tables is that the user is making an assertion about what the data looks like, hopefully with some cooperation from the FDW. It's ridiculous to suppose that we will really be able to control anything at all about the data on the foreign side, even the number of columns. But our job is to shove whatever information is present into the schema the user has specified, or throw an error. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
