Newbie DBD::Pg question

Just started on postgres a few days back and was having a few questions
about the perl interface to it. According to perldoc DBD::Pg
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Cursors

Although PostgreSQL has a cursor concept, it has not been
used in the current implementation. Cursors in PostgreSQL
can only be used inside a transaction block. Because only
one transaction block at a time is allowed, this would
have implied the restriction, not to use any nested SELECT
statements. Hence the execute method fetches all data at
once into data structures located in the frontend
application. This has to be considered when selecting
large amounts of data !
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Frankly speaking I am not sure what the Author is saying here. Anybody
cares to point me in the right direction ??? I do have to extract large
amount of data but I am not sure whether it affects me or not.

Mithun