如何有效地创建多列索引？

Question

Let's say I've coded.

create index use_index on tbl_nm (col2 ,col3 ,col4 ,col5);

would use_index be used in

select * from tbl_nm where col2="something", col5 = "something", col3="something");

Also, we should have created index by ordering the most unique on the left and the most common on the right. right?

And if I would like to order the query result should I add that column into the index too?

Answer 1

The index should be at least refs used in common queries first followed by columns search by range.

So in your example here col2 and col3 will be use this index. but because there is no col4, the searching for col5 won't be as quick. ie all col4 items with col2 and col3 matching "something" will be scanned for a matching col5 .

If you where searching for col4 rather than col5 , it would be a binary search to the required item.

Use EXPLAIN {query} to show what the index usage is.

Consideration of most unique vs least in the order of the index isn't really considered.