indexing question

Question

I have a table w/ 3 columns: name, phone, date. I have 3 indexes: 1 on phone, 1 on date and 1 on phone and date. I have the following statement:

SELECT * FROM ( SELECT * FROM people WHERE phone IS NOT NULL ORDER BY date DESC) as t GROUP BY phone

Basically, I want to get all unique phone numbers ordered by date. This table has about 2.5 million rows but takes forever to execute....are my indexes right?

UPDATE:

My EXPLAIN statement comes back with 2 rows: 1 for primary table and 1 for derived table.

It says I am using temporary and using filesort for my primary table.

For my derived table, it says my possible keys are (phone), and (phone, date) but it is using filesort.

Answer 1

you should not need the extra select * from

SELECT distinct phone
FROM people 
WHERE phone IS NOT NULL 
ORDER BY phone, date DESC

UPDATE - for all th ecolumns try this:

SELECT name, date, distinct phone     
FROM people 
WHERE phone IS NOT NULL 
ORDER BY phone, date DESC

Answer 2

I think your 3rd index is redundant, since it should already be covered by the individual indexes on the date and phone columns.

However, in your case I don't think the indexes are the real cause of the query being slow. Instead the real problem is probably the inner query that is producing huge temporary data set and as far as I know MySql is not really optimized for this.

UPDATE:
I think the following query should have the same output as yours, but would avoid the inner select:

SELECT phone, max(date) as maxDate 
FROM people 
WHERE phone IS NOT NULL 
GROUP BY phone
ORDER BY maxDate DESC

Answer 3

If you have an index (phone, date) an index of (phone) is not needed as MySQL can easily use the first index for that.

Depending on the amount of NULL phone numbers the indexes are irrelevant. You first ask the database server to get all elemenmts with phone number, if the majority has a phone number it won't care about the index then sort all of that by date, than sort again by phone and aggregate, so assuming most of your entries have a phone number you are sorting two times.

You can easily write this in a single query:

SELECT * FROM people WHERE phone IS NOT NULL GROUP BY phone, date DESC

See also the output of

EXPLAIN SELECT * FROM ( SELECT * FROM people WHERE phone IS NOT NULL ORDER BY date DESC) as t GROUP BY phone

vs.

EXPLAIN SELECT * FROM people WHERE phone IS NOT NULL GROUP BY phone, date DESC

Answer 4

I think the problem may be the "is not null" clause. Using this clause causes the db to miss the index. It has to do a full table scan to check the condition. Consider using a different default value to represent null so you can hit your index.