On 18.12.2019 13:01, Simon Riggs wrote:
I present a patch to allow READ UNCOMMITTED that is simple, useful and
efficient. This was previously thought to have no useful definition
within PostgreSQL, though I have identified a use case for diagnostics
and recovery that merits adding a short patch to implement it.
My docs for this are copied here:
In <productname>PostgreSQL</productname>'s
<acronym>MVCC</acronym>./configure
--prefix=/home/knizhnik/postgresql/dist --enable-debug
--enable-cassert CFLAGS=-O0
architecture, readers are not blocked by writers, so in general
you should have no need for this transaction isolation level.
In general, read uncommitted will return inconsistent results and
wrong answers. If you look at the changes made by a transaction
while it continues to make changes then you may get partial results
from queries, or you may miss index entries that haven't yet been
written. However, if you are reading transactions that are paused
at the end of their execution for whatever reason then you can
see a consistent result.
The main use case for this transaction isolation level is for
investigating or recovering data. Examples of this would be when
inspecting the writes made by a locked or hanging transaction, when
you are running queries on a standby node that is currently paused,
such as when a standby node has halted at a recovery target with
<literal>recovery_target_inclusive = false</literal> or when you
need to inspect changes made by an in-doubt prepared transaction to
decide whether to commit or abort that transaction.
In <productname>PostgreSQL</productname> read uncommitted mode gives
a consistent snapshot of the currently running transactions at the
time the snapshot was taken. Transactions starting after that time
will not be visible, even though they are not yet committed.
This is a new and surprising thought, so please review the attached patch.
Please notice that almost all of the infrastructure already exists to
support this, so this patch does very little. It avoids additional
locking on main execution paths and as far as I am aware, does not
break anything.
--
Simon Riggshttp://www.2ndQuadrant.com/ <http://www.2ndquadrant.com/>
PostgreSQL Solutions for the Enterprise
As far as I understand with "read uncommitted" policy we can see two
versions of the same tuple if it was updated by two transactions both of
which were started before us and committed during table traversal by
transaction with "read uncommitted" policy. Certainly "read uncommitted"
means that we are ready to get inconsistent results, but is it really
acceptable to multiple versions of the same tuple?
--
Konstantin Knizhnik
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company
