Re: XLOG's implementation details

On Nov 13, 2009, at 3:46 AM, Ozgun Erdogan wrote:

> How strong is the coupling between XLOG entries and database pages?
> That is, assume that I am inserting one row to a database table. Is
> there anyway for me to construct the related XLOG entry before calling
> the heap insert function? Or, are XLOG files conceptually binary diffs
> for database pages, moving forward in time?

The latter. The converse of your question is much more common, "How
can I reconstruct the statements that changed my database from the
xlog contents", with the same answer.

> Another question that I had relates to XLOG files and tables. If I
> have two databases and one is set up as a slave to the other, is it
> possible for the slave to have fewer tables than the master? If it is
> possible, is that because tables don't share pages?

That depends on how you implement replication. Since you ask in the
context of xlogs I'll assume you mean a warm standby slave via WAL
shipping and there the answer is no. Well, you might be able to work
something out by using record-based log shipping but even if that
could work it would take a lot of, well, work.

> Again I'm new to Postgres, and am curious about how stuff works
> underneath the covers. I figured asking this list would be the
> simplest way to figure things out.

Yep :)

Erik Jones, Database Administrator
Engine Yard
Support, Scalability, Reliability
866.518.9273 x 260
Location: US/Pacific
IRC: mage2k