SQL queries optimization - mysql

Question

I'm having trouble optimizing some sql queries that take in account datetime fields.
First of all, my table structure is the following:
CREATE TABLE info (
id int NOT NULL auto_increment,
name varchar(20),
infoId int,
shortInfoId int,
text varchar(255),
token varchar(60),
created_at DATETIME,
PRIMARY KEY(id)
KEY(created_at));
After using explain on some of the simple queries I added the created_at key, that improved most of my simple queries performance. I'm having now trouble with the following query:
SELECT min(created_at), max(created_at) from info order by id DESC limit 10000
With this query I want to get the timespan between tha last 10k results.
After using explain I get the following results:
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE info ALL NULL NULL NULL NULL 4 NULL
Any idea on how can I improve the performance of this query?

Answer 1

If you want to examine the first 10k rows ordered by id then you need to use a sub-query to achieve your goal:
SELECT MIN(created_at), MAX(created_at)
FROM (
SELECT created_at
FROM info
ORDER BY id DESC
LIMIT 10000
) tenK
The inner query gets the first 10k rows from the table, sorted by id (only the field created_at is needed). The outer table computes the minimum and maximum value of created_at from the results set generated by the inner query.
I didn't run an EXPLAIN on it but I think it says 'Using temporary' in the 'Extra' column (which is not good but you cannot do better for this request). However, 10,000 rows is not that much; it runs fast and the performance does not degrade as the table size increases.
Update:
Now I noticed this sentence in the question:
With this query I want to get the timespan between tha last 10k results.
If you want to get the value of created_at of the most recent row and the row that is 10k rows in the past then you can use two simple queries that use the index on created_at and run fast:
(
SELECT created_at
FROM info
ORDER BY id DESC
LIMIT 1
)
UNION ALL
(
SELECT created_at
FROM info
ORDER BY id DESC
LIMIT 9999,1
)
ORDER BY created_at
This query produces 2 rows, the first one is the value of created_at of the 10000th row in the past, the second one is the created_at of the most recent row (I assume created_at always grows).

Answer 2

SELECT min(created_at), max(created_at) from info order by id DESC limit 10000
The above query will give you one row containing the minimum and maximum created_at values from info table. Because it only returns 1 row, the order by and limit clauses don't come into play.
The 10000-th record from the end can be accessed with the order by & limit condition ORDER BY id DESC LIMIT 1 OFFSET 9999 (thanks #Mörre Noseshine for the correction)
So, we can write the intended query as follows:
SELECT
min_created_at.value,
max_created_at.value
FROM
(SELECT
created_at value
FROM info
ORDER BY id DESC
LIMIT 1 OFFSET 9999) min_created_at,
(SELECT
created_at value
FROM info
ORDER BY id DESC
LIMIT 1) max_created_at