Duan Pavlica writes: >maybe this is a silly question but how useful it is to create indexes >on columns containing only values 0 and 1 (true and false)?
Most of the time I'd say such an index would not be real useful. If the distribution of this column's values is equally distributed between these 2 values, then you will be accessing the rows via an index for half the values. Index accesses in such a manner are usually more expensive in a table scan. The rule of thumb that I've used and seen is about 20% of the table. So if you have evenly distributed values within the column, you'd want a cardinality of at least 5 for this guideline to be fulfilled. Especially when it comes to large queries, I've seen an index based group by take 4 times as long as a full table scan query (31 hours vs 7 hours). Most likely due to the seeks that slow down reading the table from the disk. Brad Eacker ([EMAIL PROTECTED]) -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe: http://lists.mysql.com/[EMAIL PROTECTED]