Transaction costs?

Just wondering what the cost of begin/end transaction is.

This is for pg_dump which, when restoring BLOBs, inserts multiple rows into
a temporary xref table. The sequence of events is:

Conn1: Begin
Conn1: lo_create/lo_close/lo_write.../lo_close
Conn2: Insert into xref table (which does an implicit begin/end, I think).
Conn1: Commit;

Would I get substantially better performance by doing a begin/end every
10/100/1000 rows in each connection, or is the transaction overhead low? Or
is this something I just need to test?

[eg. in Dec/RDB TX begin/end is expensive, but writing more than 1000 rows
in a TX can also be costly, so a compromise is useful]

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