Re: Transaction wraparound and read committed isolation level

On Sun, 2023-02-19 at 03:10 +0000, PG Doc comments form wrote:
> The following documentation comment has been logged on the website:
>
> Page: https://www.postgresql.org/docs/15/routine-vacuuming.html
> Description:
>
> hey guys
>
> thanks for the work you do we really appreciate it.
>
> In the transaction wraparound section this statement is misleading and got
> me really confused.
>
> “a row version with an insertion XID greater than the current transaction's
> XID is “in the future” and should not be visible to the current
> transaction“
>
> If the current transaction isolation level is read committed it absolutely
> can see rows committed by future transactions with higher XIDs.

You are right, in combination with PlanQualEval you can.

> Would be cool to add a note.
>
> this also bear the question that the wraparound isn’t really a problem with
> default isolation level but more for higher levels such as repeatable read
> and such.
>
> please correct me if my understanding is incorrect.

Wraparound can be a problem on all isolation levels. It has to do with
transaction IDs and visibility.

Yours,
Laurenz Albe