Memory usage / concept

Hi, I'm a novice in PostgreSql. I'm used to work with Oracle. I'm testing
PostgreSql (7.2) on linux RedHat 7.1 and Solaris 8. I have some problems
with large queries : for example a "select * from my_table ..." and my_table
with a size of several Gb and several Mrows. If I set the ulimit to
"unlimited", I overflow my memory and my swap; if I limit it, my query
crashes. In fact I don't know how PG works with the memory use :
with Oracle I have to manage its memory space (the SGA); is there anything
like that with PG ?
Thanks, Valerie.

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