questions about transaction and rollback

Hello,

I have questions about transaction and rollback.

Question 1:
In the postgresql.conf file of our running system,
autocommit=true. If leave it as true, can we disable
this attribute when we issue a connection to database?
Such as:

$db =
DBI->connect
(
'DBI:Pg:dbname=xxx',
'user name',
'password',
{
AutoCommit => 0
}
)

Question 2:

If we use the following command to restore database.
system("su -l postgres -c \"psql -d dbname -f
/tmp/db_dump\"");

Can command psql rollback if in the middle of restore
power failure happens?

Question 3:

During restore, if power failure happens, we want to
rollback everything having been inserted or updated to
database. In another word, if we are restoring a
database with thousands of turples changes, at the
time, we restore the last turple, power failure
happens, we want to rollback. Can postgres support
huge data changes in one transaction block? How to
configure the limit of how much we can do in one
transaction block?

Hope to hear from you soon and thank you in advance.

Best Regards

jing

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