Optimising a MySQL Query that takes ~30 seconds to run - mysql

Question

I am running a query to get the total notes input by each users between a date range. This is the query I am running:
SELECT SQL_NO_CACHE
COUNT(notes.user_id) AS "Number of Notes"
FROM csu_users
JOIN notes ON notes.user_id = csu_users.user_id
WHERE notes.timestamp BETWEEN "2013-01-01" AND "2013-01-31"
AND notes.system = 0
GROUP BY csu_users.user_id
Some notes about my setup:
The query takes between 30 and 35 seconds to run, which is too long for our system
This is an InnoDB table
The notes table is about 1GB, with ~3,000,000 rows
I'm deliberately using SQL_NO_CACHE to ensure an accurate benchmark
The output of EXPLAIN SELECT is as follows (I've tried my best to format it):
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE csu_users index user_id user_id 5 NULL 1 Using index
1 SIMPLE notes ref user_id,timestamp,system user_id 4 REFSYS_DEV.csu_users.user_id 152 Using where
I have the following indexes applied:
notes
Primary Key - id
item_id
user_id
timestamp (note: this is actually a DATETIME. The name is just misleading, sorry!)
system
csu_users
Primary Key - id
user_id
Any ideas how I can speed this up? Thank you!

Answer 1

If I'm not mistaken, by converting your timestamp to a string representation, you're loosing all advantages of the index on that column. try using timestamp values in your comparison

Answer 2

Is the csu_users table necessary? If the relationship is 1-1 and the user id is always present, then you can run this query instead:
SELECT COUNT(notes.user_id) AS "Number of Notes"
FROM notes
WHERE notes.timestamp BETWEEN "2013-01-01" AND "2013-01-31" AND notes.system = 0
GROUP BY notes.user_id
Even if that is not the case, you can do the join after the aggregation and filtering, because all the conditions are on notes:
select "Number of Notes"
from (SELECT notes.user_id, COUNT(notes.user_id) AS "Number of Notes"
FROM notes
WHERE notes.timestamp BETWEEN "2013-01-01" AND "2013-01-31" AND notes.system = 0
GROUP BY notes.user_id
) n join
csu_users cu
on n.user_id = cu.user_id