来自：http://stackoverflow.com/questions/4481388/why-does-mysql-higher-limit-offset-slow-the-query-down Scenario in short: A table with more than 16 million records [2GB in size]. The higher LIMIT offset with SELECT, the slower the query b
来自：http://stackoverflow.com/questions/4481388/why-does-mysql-higher-limit-offset-slow-the-query-down
Scenario in short: A table with more than 16 million records [2GB in size]. The higher LIMIT offset with SELECT, the slower the query becomes, when using ORDER BY *primary_key*
So
<code>SELECT * FROM large ORDER BY `id` LIMIT 0, 30 </code>
takes far less than
<code>SELECT * FROM large ORDER BY `id` LIMIT 10000, 30 </code>
That only orders 30 records and same eitherway. So it's not the overhead from ORDER BY.
Now when fetching the latest 30 rows it takes around 180 seconds. How can I optimize that simple query?
It's normal that higher offsets slow the query down, since the query needs to count off the first OFFSET
+ LIMIT records (and take only LIMIT of
them). The higher is this value, the longer the query runs.
The query cannot go right to OFFSET because,
first, the records can be of different length, and, second, there can be gaps from deleted records. It needs to check and count each record on its way.
Assuming that id is
a PRIMARY
KEY of a MyISAM table,
you can speed it up by using this trick:
<code>SELECT t.*
FROM (
SELECT id
FROM mytable
ORDER BY
id
LIMIT 10000, 30
) q
JOIN mytable t
ON t.id = q.id
</code>See this article:
MySQL cannot go directly to the 10000th record (or the 80000th byte as your suggesting) because it cannot assume that it's packed/ordered like that (or that it has continuous values in 1 to 10000). Although it might be that way in actuality, MySQL cannot assume that there are no holes/gaps/deleted ids.
So, as bobs noted, MySQL will have to fetch 10000 rows (or traverse through 10000th entries of the index on id)
before finding the 30 to return.
EDIT : To illustrate my point
Note that although
<code>SELECT * FROM large ORDER BY id LIMIT 10000, 30 </code>
would be slow(er),
<code>SELECT * FROM large WHERE id > 10000 ORDER BY id LIMIT 30 </code>
would be fast(er), and would return the same results provided that there are no missing ids
(i.e. gaps).
参考：
1.
http://timyang.net/data/key-list-pagination/
