`statistical' aggregate functions etc.

I'm a novice to postgresql, so please excuse if I'm addressing to the
wrong mailing list!

I need to specify some column functions for statistical analysis,
e.g. standard deviation or variance. After reading the user's guideI
think that's to realize with aggregates, but with those docs only I
cannot figure out how to do it -- does anybody know whether there is a
ready-for-use "statistics" package, or how to write something that's
usable in a SQL statement like this:

select date, avg(temperature), stddev(temperature)
from temperatures
where date between '01.01.1999' and '15.02.1999'
group by date ?

Other question: I'm used to use DB2/2, and DB2/2 has functions like
"month(some_date)" and "hour(some_timestamp)". Are there equivalents
in postgresql?

Many thanks in advance for help + patience!
Ulf

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