postgres

brew install postgres

initdb /usr/local/var/postgres

Or start manually with:

pg_ctl -D /usr/local/var/postgres -l /usr/local/var/postgres/server.log start

And stop with:

pg_ctl -D /usr/local/var/postgres stop -s -m fast

If you want to install the postgres gem, including ARCHFLAGS is recommended:

env ARCHFLAGS="-arch x86_64" gem install postgres