Re: designing time dimension for star schema

On Mon, Feb 10, 2014 at 9:20 AM, CS DBA <cs_dba@consistentstate.com> wrote:
> I've done a lot of DSS architecture. A couple of thoughts:
>
> - in most cases the ETL process figures out the time id's as part of the
> preparation and then does bulk loads into the fact tables
>       I would be very concerned about performance of a trigger that
> fired for every row on the fact table
>
>       you mention you want to do data streaming instead of bulk loads,
> can you elaborate?

We have processes inserting data from log files as they are written.

> - When querying a star schema one of the performance features is the
> fact that all joins to the dimension tables are performed via a numeric
> key, such as:
> "select * from fact, time_dim, geo_dim
>    where fact.time_id = time_dim.time_id..."
>
> In the case of this being a timestamp I suspect the performance would
> take a hit, depending on the size of your fact table and the
> scope/volume of your DSS queries this could easily be a show stopper
> based on the assumption that the database can do a numeric binary search
> much faster than a timestamp search

I guess I was hoping the extra 4 bytes from a timestamp, compared to a
bigint, wouldn't be too significant yet I didn't consider postgres
might do a binary search faster on an integer type than a timestamp.
Even with 1 billion rows, but maybe that's wishful thinking.  Maybe a
regular integer at 4 bytes would be good enough.  I would estimate a
query would touch up to an order of 1 million rows at a time.

Regards,
Mark