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PostgreSQL itself does not require anything extra, and includes the Pl/Python language (uses the system Python). Initial database setup is done for you if you have never installed Postgres before.
The PostgreSQL Client does not include the server. This is handy if you just need to connect to another computer that is running a Postgres server. Also included are the client libraries and headers, so developers don't have to install the full Postgres to add Postgres support to their software. The full Postgres package includes Postgres Client, so there is no need to install the client if you already installed the server.
The PostGIS extension requires the GEOS 3 and PROJ frameworks (also in the GDAL Complete package). The WKTRaster extension requires GDAL Complete 1.8 and PostGIS. The pgRouting extension does not require anything extra, though it can use PostGIS. All extensions require the Postgres server and won't function with the client-only package.
If you want to update many parts at once, ie Postgres, PostGIS and the system, here is a recommended order of updates:
NOTE: I've had reports of the installer not initializing the data cluster, which results in Postgres not starting. It seems to be random, and I haven't figured out the cause yet. If Postgres won't start (you get an error "could not connect to server: No such file or directory" when trying to connect with
psql), try manually initializing the cluster with:
/usr/local/pgsql-9.1/bin/initdb -U postgres \ -D /usr/local/pgsql-9.1/data --encoding=UTF8 --locale=en_US"
Or see the Locales and Encodings readme if you need a different locale or encoding.
Then stop and start Postgres as specified in the readme.