My gut feeling is that it's fairly well known, but few people use it. Including yours truly.

Then again, I have rarely run into performance issues with database engines. Either the ones I've had contact with (SQLite, MSSQL, Postgres) are very good at what they do (which I suspect they are), or my requirements were too puny for them to break a sweat (which I also suspect to be the case).

I did use MSSQL Studio's query analyzer once to fix a performance issue and was able to get the query in question from nearly an hour down to under a second. triumphant grin I don't remember the details of the query, but the solution was to use a hash join.

I'm glad you said it because I was about to say the exact same thing.