Different Linux distros have their preferred place of where stuff goes and of course the default location on windows is completely different from that too. So there isn't really one default location where you can find PostgreSQL data cluster. Of course user's can pick their
locations as well. So what is a casual DBA supposed to do?
The pg_settings table
PostgreSQL has a convenient system table view called pg_settings that stores a lot of information. It stores the location of the data cluster, the pg_hbafile and other conf files.
In additon to that you can interogate it to find out information you will find in the postgresql.conf file. Why sift thru that postgresql.conf file (assuming you can already query your postgresql server) when you can find the answers you are looking
for with an SQL query?
This question is one that has come up a number of times in PostGIS newsgroups worded in many different ways. The situation is that if you use a function a number of times
not changing the arguments that go into the function, PostgreSQL still insists on recalculating the value even when the function is marked IMMUTABLE. I have tested this on
8.2 and 8.3 with similarly awful results.
This issue is not so much a problem if function calculations are fast, but spatial function calculations relative to most other functions you will use are pretty
slow especially when dealing with large geometries. As a result your query could end up twice as slow. Even setting the costs of these functions to relatively high does not help the situation.
To demonstrate here is a non-PostGIS version of the issue that everyone should be able to run and demonstrates its not a PostGIS only issue.