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