It is a general rule that indexes for columns with low cardinality are not worth it, often making queries more expensive than they would be without said index. binary columns all suffer from this.
- michael dykman On Sat, Aug 27, 2011 at 4:52 PM, Dave Dyer <ddyer-my...@real-me.net> wrote: > > The "innocuous change" was to add an index for "is_robot" which is true > for 6 out of 20,000 records and null for the rest. > > My complaint/question/observation is not how to optimize the query > that went awry, but to be alarmed that a venerable and perfectly > serviceable query, written years ago and ignored ever since, suddenly > brought the system crashing down after making a seemingly innocuous > change intended to make a marginal improvement on an unrelated query. > > I had previously believed that tinkering the schema by adding indexes was a > safe activity. It's as though I add a shortcut to my regular commute > and caused a massive traffic jam when the entire traffic flow tried to > follow me. > > (Both tables are "ok" according to analyze table) > > > -- > MySQL General Mailing List > For list archives: http://lists.mysql.com/mysql > To unsubscribe: http://lists.mysql.com/mysql?unsub=mdyk...@gmail.com > > -- - michael dykman - mdyk...@gmail.com May the Source be with you.
