A batch loader that writes N rows per transaction, each row inside a BEGIN ... EXCEPTION block so one bad row does not abort the batch, is usually capped at a fixed number of rows (e.g. 50) to stay under PostgreSQL's 64 cached subtransaction XIDs per backend. When one input line starts writing several records plus links between them, each in its own exception block, a 50-row batch can open 250+ subtransactions. Lookups by other sessions then slow down (subtrans overflow) with no error.