Tino Wildenhain wrote:
Google or your favourite search engine helps :-)

http://www.postgresql.org/files/documentation/books/aw_pgsql/node71.html

is among the first results.

Unfortunately those docs are quite out of date. This page is better:

http://developer.postgresql.org/docs/postgres/datatype-oid.html

(This describes the behavior that will be the default in 8.1)

Specifically:

Object identifiers (OIDs) are used internally by PostgreSQL as primary keys for various system tables.... The oid type is currently implemented as an unsigned four-byte integer. Therefore, it is not large enough to provide database-wide uniqueness in large databases, or even in large individual tables. So, using a user-created table's OID column as a primary key is discouraged. OIDs are best used only for references to system tables.

-Neil


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