[bananas]

The only problem with a limited number of custom fields attached to the table is that they aren't necessarily optimised for sorting, might extend past the row size limit of database tables (this is a pain!), can't be reliably typed and a client will always want one more.

EAV is a sort of inner platform thing I agree, but the correct solution i.e a document store with full field level indexing that works with enterprise loads doesn't exist (yet). CouchDB was promising on that front but didn't go all the way.

XML is fine. Just don't stick it in database columns (my favourite chunk of pain!)

[wiredfool]

I like HStore a lot better than XML for postgres. And it's Waaaaay better than the wide table model.

Way back when, I did a data model that added columns for arbitrary data fields that the users wanted in PG, and wound up with a wide table model. PG can store a surprisingly large number of fields, I think it got up to the mid to high hundreds after years of this. At the time, hstore wasn't there, xml was either not there yet, or just recently added. And the queries for EAV looked surprisingly awful, especially when added to the not exactly straightforward queries we were doing on the events.

It wound up being an extremely large, extremely sparse table, with some fields having 100% usage, most having >>.01%, and a few getting used in the 1% range. On the plus side, it was possible to index any of the fields, which was especially useful with functional or 2 column indexes.

If I had to do it again, I'd be on hstore, or maybe hstore/json. It wasn't pretty but it wasn't the fatal flaw in that startup.

[arethuza]

"XML is fine. Just don't stick it in database columns"

I've seen systems that used XML in a database column and were perfectly sensible, I've also seen systems that did terrible things with XML in a database (e.g. using complex XML string as part of a query).