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In Postgres 11, according to documentation, txid_current() should return biginteger but I'm getting small integer that doesn't look like is extended by epoch counter. My txid looks like this: 1123

"The internal transaction ID type (xid) is 32 bits wide and wraps
around every 4 billion transactions. However, these functions 
export a 64-bit format that is extended with an “epoch” counter
so it will not wrap around during the life of an installation."

https://www.postgresql.org/docs/11/functions-info.html

Am I interpreting documentation incorrectly?

  • How do you know 1123 is not a bigint value? Try with: select pg_typeof(txid_current()); – McNets Jun 28 at 13:02
  • The txid supposed to be extended with an epoch counter. I don't see it extended by epoch anywhere. Even if epoch is added to an ID and the sum is displayed as txid I still don't understant how it is extended since txids are serial numbers without regard to a time interval between transactions. – Jimski Jun 28 at 17:22
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The internal transaction counter in PostgreSQL is 4 bytes wide, but txid_current() will return a bigint that keeps increasing even after transaction wraparound. Think of it as an “external transaction ID”.

0

Postgres has an internal 32bit xid counter which is different than the value returned by txid_current().

The txid_current() returns 64bit (bigint) in which the high bits are an epoch increment which happens once per xid wrap around.

So at every wraparound the epoch kicks in and txid_current() keeps incrementing. Thats why at a fresh launch of Postgres the returned txid looks like a regular integer and the epoch element is not apparent.

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