gist geometric equality operator - for geometry uniqueness constraints

[robe]

The geometric equality operator we used to have in 1.3 (~ would be even more useful now. We can't call it ~= since that has already changed definition so much and is now just a bounding box equality (and not even that much because our bounding boxes are floats instead of the double precision of the geometry)

Something like == might be a possible choice for bringing back the good old geometric equality ~= (NOTE this requies it have a RECHECK on it.

The main benefit would be to be able to use exclusion constraints (introduced in PostgreSQL 9.0) to enforce geometry uniqueness. Right now there is no guaranteed way of doing this and EXCLUSION CONSTRAINTS won't work with functions (so ST_OrderingEquals is out of the question) — they have to be operators.

​http://www.postgresql.org/docs/9.0/static/ddl-constraints.html ​http://thoughts.j-davis.com/2010/09/25/exclusion-constraints-are-generalized-sql-unique/