snapshots

Hello.

Could somebody explain me the mechanism in the backend,
which is responsible for the followings. (I tried to
look around snapshots, but couldnt figure out th answer).

In a transaction, isol. read comitted, a select from a
table can see the comitted changes by others, but
a previously declared cursor can not.

A few lines in src/backend/tcop/pquery.c runs when
a DECLARE cursor is issued:

if (isRetrieveIntoPortal)
{
PortalExecutorHeapMemory = NULL;

ProcessPortal(intoName,
[...]
return;
}

, but I cannot see neither in these, nor in ProcessPortal that
where does it fix the set of rows being visible.

Please help to find it out.

Thanks,
Baldvin