800MB is correct, yes... There are usually only 10-30 postgres processes active (imagine 5-10 people working on the web front-end while cron jobs access the db occasionally). Very few queries can use such large amounts of memory for sorting, but they do exist.
But remember that means that if you have 4 people doign 2 sorts each at the same time, postgres will use 6.4GB RAM maximum. The sort_mem parameter means that if a sort is larger than the max, it will be done in disk swap.
Chris
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