On Thu, Nov 21, 2013 at 9:57 AM, Dario Taraborelli <dtaraborelli@wikimedia.org> wrote:

I tried uploading a cohort from a recent A/B test (1,780 unique user_id’s). The async validation took about 5 minutes to complete.

If I create a temporary table with the data in my CSV and run a join with the user table against a slave, the query to validate that these users exist takes about 400ms if I use user_id (primary key in enwiki.user) and about 3s using user_name (unique in enwiki.user). 

What’s the reason why it takes so long to validate a cohort in the application?

My understanding is that this is due to Labs being slow compared to stat1? 

I don't think labs is that much slower though, we're talking orders of magnitude here.  So, I think the reason is that currently it's validating one user at a time.  Since for each record I have to check against a potential user_id and user_name match, this takes forever.

Two ways to make it much faster:

* batch every X users and do a where user_id in (...) or user_name in (...) query instead of checking each one

* create temporary tables just like Dario did

The problem is that cohorts can have users from multiple projects.  That makes both approaches harder, but should still be doable.  The reason I haven't done this yet is that when we scheduled 818 we broke out the performance issue and agreed we'd work on it later.  Sounds important though, I'll look at it now.