Optimize mysql query using indexes

Question

I have a problem with this query:

SELECT DISTINCT s.city, pc.start, pc.end 
FROM postal_codes pc LEFT JOIN suspects s ON (s.postalcode BETWEEN pc.start AND      pc.end) 
WHERE pc.user_id = "username" 
ORDER BY pc.start

Suspect table has about 340 000 entries, there is a index on postalcode, I have several users, but this individual query takes about 0.5s, when I run this SQL with explain, I get something like this: http://my.jetscreenshot.com/7536/20111225-myhj-41kb.jpg - does these NULLs mean that the query isn't using index? The index is a BTREE so I think this should run a little faster.

Can you please help me with this? If there are any other informations needed just let me know.

Edit: I have indexes on suspects.postalcode, postal_codes.start, postal_codes.end, postal_codes.user_id.

Basically what I'm trying to achieve: I have a table where each user ID has multiple postalcode ranges assigned, so it looks like:

user_id | start | end

Than I have a table of suspects where each suspect has an address (which contains a postalcode), so in this query I'm trying to get postalcode range - start and end and also name of the city in this range.

Hope this helps.

Answer 1

Whenever left join is used all the records of the first table are picked up rather than the selection on the basis of index. I would suggest to using an inner join. Something like in the below query.

select distinct 
  s.city, 
  pc.start, 
  pc.end 
from postal_codes pc, suspect s 
where 
  s.postalcode between (select pc1.start, pc1.end from postal_code pc1 where pc1.user_id = "username" ) 
  and pc.user_id = "username"
order by pc.start

Answer 2

It's using only one index, and not for the fields involved in the join. Try creating an index for the start and end fields, or using >= and <= instead of BETWEEN

Answer 3

Not 100% sure, but this might be relevant:

Sometimes MySQL does not use an index, even if one is available. One circumstance under which this occurs is when the optimizer estimates that using the index would require MySQL to access a very large percentage of the rows in the table. (In this case, a table scan is likely to be much faster because it requires fewer seeks.) However, if such a query uses LIMIT to retrieve only some of the rows, MySQL uses an index anyway, because it can much more quickly find the few rows to return in the result.

So try testing with LIMIT , and if it uses the index then, you found your cause.

Answer 4

I have to say I'm a little confused by your table naming convention, I would expect the "suspect" table to have a user_id not the postal_code, but you must have your reasons. If you were to leave this query as it is, you can add an index on postal_code (star,end) to avoid the complete table scan.

Answer 5

I think you can restructure your query like following,

SELECT DISTINCT s.city, pc1.start, pc1.end FROM 
(SELECT pc.start and pc.end from postal_codes pc where pc.user_id = "username") as pc1,    Suspect s
WHERE s.postalcode BETWEEN pc1.start, pc1.end ORDER BY pc1.start

your query is not picking up the index on s table because of left join and your between condition. Having an Index in your table doesn't necessarily mean that it will be used in all the queries.

Answer 6

尝试FORCE INDEX 。