On Wed, Feb 10, 2010 at 4:48 PM, Dave Crooke <dcro...@gmail.com> wrote:
> On Wed, Feb 10, 2010 at 5:30 PM, Jon Lewison <jlewis...@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.
>
Yes, absolutely. It's not unusual to see the UNDO tablespace increase in
size by several gigs for a large bulk load.
Speaking of rollback segments I'm assuming that since all storage for
non-visible row versions is in the main table that PostgreSQL has no
equivalent for an ORA-01555.
- Jon
