Re: Read Uncommitted - Mailing list pgsql-hackers

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 Riggs                http://www.2ndQuadrant.com/
PostgreSQL Solutions for the Enterprise

-- 
Konstantin Knizhnik
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company