From: Dave Crooke <dcrooke@gmail.com>
To: Jon Lewison <jlewison1@gmail.com>
Cc: rama <rama.rama@tiscali.it>
Cc: pgsql-performance@postgresql.org
Subject: Re: perf problem with huge table
Date: Wed, 10 Feb 2010 17:48:09 -0600
Message-ID: <ca24673e1002101548k2513c23k302ebeca77070b2a@mail.gmail.com> (raw)
In-Reply-To: <7ac84c51002101530q59fa87c1w205fdf5243d9d69a@mail.gmail.com>
References: <BDE4B02A-24A4-46C1-9C76-63AA5DD57B40@tiscali.it>
<ca24673e1002101516wa2545aek80cbe88df935698@mail.gmail.com>
<7ac84c51002101530q59fa87c1w205fdf5243d9d69a@mail.gmail.com>
On Wed, Feb 10, 2010 at 5:30 PM, Jon Lewison <jlewison1@gmail.com> wrote:
>
>
> Just a nit, but Oracle implements MVCC. 90% of the databases out there do.
>
Sorry, I spoke imprecisely. What I meant was the difference in how the rows
are stored internally .... in Oracle, the main tablespace contains only the
newest version of a row, which is (where possible) updated in place -
queries in a transaction that can still "see" an older version have to pull
it from the UNDO tablespace (rollback segments in Oracle 8 and older).
In Postgres, all versions of all rows are in the main table, and have
validity ranges associated with them ("this version of this row existed
between transaction ids x and y"). Once a version goes out of scope, it has
to be garbage collected by the vacuuming process so the space can be
re-used.
In general, this means Oracle is faster *if* you're only doing lots of small
transactions (consider how these different models handle an update to a
single field in a single row) but it is more sensitive to the scale of
transactions .... doing a really big transaction against a database with an
OLTP workload can upset Oracle's digestion as it causes a lot of UNDO
lookups, PG's performance is a lot more predictable in this regard.
Both models have benefits and drawbacks ... when designing a schema for
performance it's important to understand these differences.
> I find partitioning pretty useful in this scenario if the data allows is.
> Aging out data just means dropping a partition rather than a delete
> statement.
>
>
Forgot to say this - yes, absolutely agree .... dropping a table is a lot
cheaper than a transactional delete.
In general, I think partitioning is more important / beneficial with PG's
style of MVCC than with Oracle or SQL-Server (which I think is closer to
Oracle than PG).
Cheers
Dave
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-performance@postgresql.org
Cc: dcrooke@gmail.com, jlewison1@gmail.com, rama.rama@tiscali.it
Subject: Re: perf problem with huge table
In-Reply-To: <ca24673e1002101548k2513c23k302ebeca77070b2a@mail.gmail.com>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox