Re: Read Uncommitted
| От | Simon Riggs |
|---|---|
| Тема | Re: Read Uncommitted |
| Дата | |
| Msg-id | CANP8+jLw2pgqwe2OPozVxO0e+O5JySF5MLnFRX8hua5kqYn3Nw@mail.gmail.com обсуждение исходный текст |
| Ответ на | Re: Read Uncommitted (Tom Lane <tgl@sss.pgh.pa.us>) |
| Ответы |
Re: Read Uncommitted
|
| Список | pgsql-hackers |
On Wed, 18 Dec 2019 at 14:06, Tom Lane <tgl@sss.pgh.pa.us> wrote:
Simon Riggs <simon@2ndquadrant.com> writes:
> I present a patch to allow READ UNCOMMITTED that is simple, useful and
> efficient.
Won't this break entirely the moment you try to read a tuple containing
toasted-out-of-line values? There's no guarantee that the toast-table
entries haven't been vacuumed away.
I suspect it can also be broken by cases involving, eg, dropped columns.
There are a lot of assumptions in the system that no one will ever try
to read dead tuples.
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.
So this won't happen with the proposed patch.
The fact that you can construct a use-case in which it's good for
something doesn't make it safe in general :-(
I agree that safety is a concern, but I don't see any safety issues in the patch as proposed.
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