What constitutes a transaction when reading BLOBs?

Hi,

We have a read-only database with large BLOBs, and the field
'xact_commit' of pg_stat_database is incremented by thousands every minute.
The number of concurrent connections is about 5-10.
What is considered a transaction in general and when BLOBs are involved
and how does it affect the autovacuum?

Thank you,
Irina

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Irina Sourikova
Brookhaven National Laboratory phone: +1-631-344-3776
Physics Department, Bldg 510 C fax: +1-631-344-3253
Upton, NY 11973-5000 email: irina(at)bnl(dot)gov