In the last episode (Feb 18), Renars Jeromans said: > This is not an urgent problem, but it has always intrigued me... It's > simplified case, but it makes the point. Let us assume that we have a > table > > create table T (id unsigned int unique, name char(10)); > create index i_name on T(name); > > Let's insert into it say 5 mil rows with name field being just say 5 > distinct values AAA, BBB, CCC, DDD, EEE. > > Now the question. Why a query like > > select distinct name from T; > > takes about 3 sec to return just a bunch of rows? As I understand it, > index contains all these 5 values, so just simple lookup into index > should take fractions of a second. Can anyone comment on this? MySQL > team?
You need to walk the entire index to make sure you have all the values. There might be a single "AAB" inbetween those million "AAA"'s and million "BBB"'s. It requires a full index scan, which is usually a lot faster than a full table scan, but it is still not instantaneous. In your example it won't be any quicker since your table only has two rows, but in the usual case where a row may be a couple hundred bytes long there is a bigger difference. -- Dan Nelson [EMAIL PROTECTED] --------------------------------------------------------------------- Before posting, please check: http://www.mysql.com/manual.php (the manual) http://lists.mysql.com/ (the list archive) To request this thread, e-mail <[EMAIL PROTECTED]> To unsubscribe, e-mail <[EMAIL PROTECTED]> Trouble unsubscribing? Try: http://lists.mysql.com/php/unsubscribe.php