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On 05/23/07 19:17, Chris Browne wrote:
> harpreet.dhaliwal01@gmail.com ("Harpreet Dhaliwal") writes:
>> I was just wondering if Vacuum Db in postgresql is somehow superior
>> to the ones that we have in other RDBMS.
>
> The thing that is more akin to VACUUM, in Oracle's case, is the
> rollback segment. In Oracle, Rollback segments are areas in your
> database which are used to temporarily save the previous values when
> some updates are going on.
>
> In the case of Oracle, if a transaction rolls back, it has to go and
> do some work to clean up after the dead transaction.
>
> This is not *exactly* like PostgreSQL's notion of vacuuming, but
> that's the nearest equivalent that Oracle has.
That's the only other way to do it, no?
(Rdb/VMS has dynamically-created [made when a process attaches to
the db] Recovery Unit Journal files that store the record before-
images.)
> The Oracle InnoDB product also has the notion of rollback segments; if
> you use InnoDB tables with MySQL, the rollback functionality has much
> the same behaviour as Oracle.
>
> Note that in the case of PostgreSQL, the MVCC behaviour (which
> requires VACUUMing) has the merit that COMMIT and ROLLBACK both have
> near-zero costs; in either case, the cost is merely to mark the
> transaction as either committed or failed. Data doesn't have to be
> touched at time of COMMIT/ROLLBACK; any costs that need to be paid are
> deferred to VACUUM time.
So it's not "near-zero cost", it's "deferred cost".
- --
Ron Johnson, Jr.
Jefferson LA USA
Give a man a fish, and he eats for a day.
Hit him with a fish, and he goes away for good!
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