MySQL and materialized views
I'm working on alternative strategies to make the use and maintenance of a multi-terabyte data warehouse implementation tolerably fast. For example, it's clear that a reporting query on a 275-million row table is not going to be fun by anyone's definition, but that for most purposes, it can be pre-processed to various aggregated tables of significantly smaller sizes.
However, what is not obvious is what would be the best strategy for creating those tables. I'm working with MySQL 5.0 and Business Objects' Data Integrator XI, so I have a couple of options.
I can just CREATE TABLE ... SELECT ... to see how things work out. This approach is simple to try, but essentially unmaintanable; no good.
I can define the process as a BODI data flow. This is good in many respects, as it creates a documented flow of how the aggregates are updated, is fairly easy to hook up to the workflows which pull in new data from source systems, and allows monitoring of the update processes. However, it's also quite work intensive to create all those objects with the "easy" GUIs in comparison to just writing a a few simple SQL statements. There are also some SQL constructs that are horribly complicated to express in BODI; in particular, COUNT(DISTINCT ..) is ugly.
Or I could create the whole process with views on the original fact table, with triggered updates of a materialized view table in the database. It would still be fairly nicely documentable, thanks to the straightforward structure of the views, and very maintanable, as the updates would be automatic. A deferred update mechanism with a trigger keeping track of which part of the materialized view needs update and a periodic refresh over a stored procedure would keep things nicely in sync. MySQL 5.0 even has all of the necessary functionality.
Except.. It's only there in theory. The performance of views and triggers is so horrible that any such implementation would totally destroy the usability of the system. MySQL's views only work as statement merge when there is a one-to-one relationship between base table and view rows, or in other words, the view can not contain SUM(), AVG(), COUNT() or any of the other mechanisms which would have been the whole point of the materialized view in question. It will fall back to a temp table implementation in these cases, and creating a GROUP BY temp table over 275 million rows without using the WHERE BY clause is pure madness.
In addition, defining any triggers, however simple, slow bulk loads to the base tables by an order of magnitude. I could of course still work around triggers by implementing the equivalent logging in each BODI workflow and create the materialized views and a custom stored proc to update each one, but having a view there in between was the only way to make this approach maintainable. Damn, there goes that strategy.