I have a Table with about 800,000 record DB Version : 3. 2. 7 The Sql looks like select x, y, sum(z)/1000 as bw from aa where a=1 and b=1 and c =1 and d= 6 group by x, y having count(*) > 1 order by bw desc limit 10
Column "d" is all set to 6 1/3 of total record is (a=1,b=1,c=1) 1/3 of total record is (a=2,b=2,c=2) 1/3 of total record is (a=3,b=3,c=3) Column x,y may have random value ? I just test the above Sql under different index (each time, I just create one index and drop another) 1) create index 1 on aa (d) 2) create index 2 on aa (a,b,c,d) Expect : I think Using second index should be much faster than using first index. Actually That two index almost give the same performance. both of them will take about 15 seconds Why? thanks in advice :) __________________________________________________ 赶快注册雅虎超大容量免费邮箱? http://cn.mail.yahoo.com