By my (admittedly limited) understanding of MVCC, one long running transaction that includes an insert will not block other inserts to the same table. The worst you'd see in that situation is if you were using a sequence for IDs, an ID might be grabbed from the sequence at the beginning of transaction A, then another ID grabbed and inserted into the table by transaction B, and then when transaction A finally commits, the order of the IDs won't match the chronological order of the commits.