I'm trying to get a better understanding of how XID wraparound affects the results of the query to find the snapshot xmin (that is txid_snapshot_xmin(txid_current_snapshot()) or pg_snapshot_xmin(pg_snapshot_current()) after Postgres 13) and the stored xmin value in Postgres tuples:

My understanding of XID wraparound is :

  1. After 4 billion transactions, the xmin value wraps around (starts from 0, 1, 2...). This is internally represented as an xid, which is essentially a 32 bit integer.
  2. However, the results of txid_snapshot_xmin(txid_current_snapshot()) or pg_snapshot_xmin(pg_snapshot_current()), according to the documentation, return an xid8 which increases monotonically, and never wraps around. ref
  3. The snapshot value in step 2 has an epoch value associated with it which can be calculated as txid_snapshot_xmin(txid_current_snapshot()) >> 32. This essentially represents the number of times wraparound has occurred. More importantly, if this value is different from a previous queried value, this means that wraparound has occurred. Conversely, by executing txid_snapshot_xmin(txid_current_snapshot()) % 2^32 one can get the xmin value that would is assigned to a row (the 32-bit value)

I'm trying to simulate this behavior to verify my theory, but I don't currently have access to a high volume database (with > 4 billion transactions).

Is there a good way to simulate a large volume of transactions on a test DB to see this wraparound in action? That is, without actually executing 4B transactions?

I'm thinking that maybe there is some internal state/counter of the current number of transaction that I can modify, but I haven't had any luck with this.

  • Please don't ask the same question in multiple Stackexchange sites. Answered on Stackoverflow. Apr 15 at 3:04
  • Apologies, I will not do so in the future.
    – Dev K
    Apr 20 at 0:02


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