Challenges preventing us moving to 64 bit transaction id (XID)?

From: Tianzhou Chen <tianzhouchen(at)gmail(dot)com>
To: pgsql-hackers(at)postgresql(dot)org
Subject: Challenges preventing us moving to 64 bit transaction id (XID)?
Date: 2017-06-05 08:49:34
Message-ID: DA1E65A4-7C5A-461D-B211-2AD5F9A6F2FD@gmail.com
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Hi Pg Hackers,

XID wraparound seems to be quite a big concern and we introduce changes like “adding another frozen bit to each page” [http://rhaas.blogspot.com/2016/03/no-more-full-table-vacuums.html <http://rhaas.blogspot.com/2016/03/no-more-full-table-vacuums.html> to tackle this. I am just wondering what’s the challenges preventing us from moving to 64 bit xid? This is the previous thread I find https://www.postgresql.org/message-id/CAEYLb_UfC%2BHZ4RAP7XuoFZr%2B2_ktQmS9xqcQgE-rNf5UCqEt5A%40mail.gmail.com <https://www.postgresql.org/message-id/CAEYLb_UfC+HZ4RAP7XuoFZr+2_ktQmS9xqcQgE-rNf5UCqEt5A@mail.gmail.com>, the only answer there is:


The most obvious reason for not using 64-bit xid values is that they
require more storage than 32-bit values. There is a patch floating
around that makes it safe to not forcibly safety shutdown the server
where currently it is necessary, but it doesn't work by making xids
64-bit.

"

I am personally not quite convinced that is the main reason, since I feel for database hitting this issue, the schema is mostly non-trivial and doesn’t matter so much with 8 more bytes. Could some postgres experts share more insights about the challenges?

Thanks
Tianzhou

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