http://www.postgresql.org/files/documentation/faqs/FAQ.html#4.16
The FAQ on oids is out of date. It claims that they are globally unique
and that no one has ever rolled over the oid counter. This should be
changed to actively discourage their use.
Kris Jurka
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TIP 9: the planner will ignore your desire to choose an index scan if your
joining column's datatypes do not match
