Forum OpenACS Q&A: Response to something like vacuum analyze or compact database for Oracle?

Don, do you know how PG behaves if, while retrieving rows during the OPEN, rows change? For example, if I OPEN a query like SELECT name, balance FROM accounts, while someone takes $500 from balance on row 1 (after I read it), and adds to to balance on row 5000000 (before I read it). Will PG see the change on row 5000000? Will it lock others out until it's done getting all 5000000 rows?