How does the update process work, and how is data appended?

[FaFre]

I use osm2pgsql together with the flex output. My Lua script processes and remixes the data with other data sources. After the insert I use Postgres triggers to further process geometries (I'm aware of the new features in 1.7, but the current function set is not enough for my scenario).

At some point I want to append diff files to my database. How exactly does o2p handle the database updates? I could not find detailed information about that in the documentation (as most users don't have to care about this I guess).

Following things are important to know, to keep the database in shape with my triggers:

• How does o2g actually know when data is outdated? Does it only rely on the diff files, or are also SELECT's issued to the database to compare data?
• To update the data, o2p uses UPDATE or DELETE/INSERT statements?

It would be great to know about that, and also if the currently used update method is likely to change in the near future.

[joto]

The details are not documented on purpose, because they might change at any time. To find out what osm2pgsql is doing internally, you can use the --log-sql option though. :-)