what causes xact_rollback for a database in pg_stat_database to increment?

We've been seeing over 200k rollbacks being recorded for a database at certain hour overnight but can't see the corresponding ROLLBACK statements being recorded in postgresql db logs, even though our log_statement is set to ALL.

I did some tests and confirmed that xact_rollback count increments in scenarios below and am looking further at those.

1. a ROLLBACK is issued explicitly or implicitly(if a transaction or pl/pgsql block/function raises an exception or was aborted)

2. it doesn't reflect the number of records being rolled back

3. even if ROLLBACK statement results in WARNING like "there is no transaction in progress", it increments

Does anyone know on top of his/her head any (other) explanation for such unaccounted for rollbacks?

Thanks,
Patricia

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