> From: mos <mo...@fastmail.fm> > > > At 10:34 AM 10/24/2011, you wrote: >> select id from table order by rand() limit 1; >> is doing as example a dumb temporary table with the full size > > Because it has to sort the entire table, then it returns the one row. This of > course is extremely inefficient. :)
That is absolutely incredible and counter-intuitive, and (as you say) extremely inefficient! This is used everywhere. Perhaps it is one of the biggest "anti-patterns" in SQL. I just checked two different SQL "cookbook" sites, and they both recommend ORDER BY RAND(). I just googled around a bit, and found that putting RAND() in the WHERE clause is very efficient: SELECT id FROM table WHERE RAND() < 0.01 LIMIT 1 The comparison constant can be optimized for the number of rows you have. The above returns the first record of 1% of the table. If you have a million rows, you might want to bump that to something like 100 parts per million or so. But really, folks, this is something so ubiquitous and so recommended, why can't the query optimizer look out for ORDER BY RAND() and simply skip the table sort and just grab some record? (Hopefully using something better than Knuth's LCRNG...) ---------------- Learning to think wholistically requires an overriding, or reversal, of much of the cultural heritage of the last few hundred years. -- David Holmgren :::: Jan Steinman, EcoReality Co-op :::: -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe: http://lists.mysql.com/mysql?unsub=arch...@jab.org
