Обсуждение: snapshot question
Per the following link: https://www.sobyte.net/post/2022-08/postgres-snapshot/
To get a snapshot we need to know xmin, xmax and xip. my question is why do we need them? Since every tuple in a page has t_xmin and t_xmax based on the page layout(https://www.postgresql.org/docs/current/storage-page-layout.html), if we has a snapshot at xip 12345, couldn't we just check every tuple whose t_xmin is lower than 12345 and already committed?
Thanks,
Qihua
On Wed, Dec 7, 2022 at 10:20:09AM +0800, qihua wu wrote: > Per the following link: https://www.sobyte.net/post/2022-08/postgres-snapshot/ > To get a snapshot we need to know xmin, xmax and xip. my question is why do we > need them? Since every tuple in a page has t_xmin and t_xmax based on the page > layout(https://www.postgresql.org/docs/current/storage-page-layout.html), if we > has a snapshot at xip 12345, couldn't we just check every tuple whose t_xmin is > lower than 12345 and already committed? See, slides 9-11: https://momjian.us/main/presentations/internals.html#mvcc xip is a list of transaction that were open at the time the snapshot was taken. If the transaction commits after the snapshot is taken, it is still invisible. -- Bruce Momjian <bruce@momjian.us> https://momjian.us EDB https://enterprisedb.com Embrace your flaws. They make you human, rather than perfect, which you will never be.