On 06/05/2017 11:49 AM, Tianzhou Chen wrote:
Hi Pg Hackers,

XID wraparound seems to be quite a big concern and we introduce
changes like “adding another frozen bit to each page”
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
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?

That quote is accurate. We don't want to just expand XIDs to 64 bits, because it would significantly bloat the tuple header. PostgreSQL's per-tuple overhead is already quite large, compared to many other systems.

The most promising approach to tackle this is to switch to 64-bit XIDs in in-memory structures, and add some kind of an extra epoch field to the page header. That would effectively give you 64-bit XIDs, but would only add one a field to each page, not every tuple.

- Heikki

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