| From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
|---|---|
| To: | Simon Riggs <simon(at)2ndquadrant(dot)com> |
| Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org> |
| Subject: | Re: Read Uncommitted |
| Date: | 2019-12-18 17:35:28 |
| Message-ID: | CA+TgmoabdBCUL-kujCnLZ1kGmLny2aKgHXaQHZZm42VD_GYA7g@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Wed, Dec 18, 2019 at 10:18 AM Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> This was my first concern when I thought about it, but I realised that by taking a snapshot and then calculating xmin normally, this problem would go away.
Why? As soon as a transaction aborts, the TOAST rows can be vacuumed
away, but the READ UNCOMMITTED transaction might've already seen the
main tuple. This is not even a particularly tight race, necessarily,
since for example the table might be scanned, feeding tuples into a
tuplesort, and then the detoating might happen further up in the query
tree after the sort has completed.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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