Re: WIP: generalized index constraints

I'm going to try to get this patch ready for the 9-15 commitfest. Here
are a few design updates:

(1) Language:

I think that the new language should be a table constraint, and I think
there's a consensus on that. The specific language I have in mind is:
 CREATE TABLE (   ...,   INDEX CONSTRAINT (<attname> <op>, ...) USING INDEX <indexname> );
 ALTER TABLE ADD INDEX CONSTRAINT (<attname> <op>, ...)   USING INDEX <indexname>;

Where <op> is the constraint operator. For example, if all <op>s are
"=" (or whatever the operator for BTEqualStragey is for that type), that
would be semantically identical to a UNIQUE constraint.

(2) Enforce while creating constraint:

To enforce the constraint while adding it to a table with existing data,
I'd treat it somewhat similar to a CHECK constraint: exclusive lock on
the table, and check every tuple. 

Note that this is going to be O( n * log n ), assuming a search time of
O( log n ) on the index. A CHECK constraint is obviously just O(n).

(3) Catalog change:

I'll need to add a column pg_constraint.constrategies of type
int2vector. This will hold the strategy numbers of the constraint
operators. For instance, if there's a 2-column index constraint on a
BTree index, and both constraint operators are "=", it will be a vector
containing [BTEqualStrategy, BTEqualStrategy].

I will add a column pg_class.relindconstraints which will be similar to
relchecks (see below).

Also, introduce a new "contype 'i'" meaning that it's an index
constraint.

The patch relies on the existence of pg_constraint.conindid, which is
already committed for 8.5.

(4) Enforce on insert procedure:

This is mostly the same as the previous patch. However, I think I need
to avoid catalog lookups in the index insert path, which is a problem
Tom pointed out in the deferrable unique constraints discussion.

My plan is to make it a part of ResultRelInfo and initialize it in a way
similar to a CHECK constraint (in ExecRelCheck() if it's not already
initialized).

I would use relindconstraints to prevent going into that path during the
common case where there is no index constraint (in particular, when
bootstrapping).

Comments, suggestions, ideas?

Regards,Jeff Davis