Simple SQL Query takes 10-20 times more with "ORDER BY" - mysql

Question

Simple SQL query takes 10 to 20 times longer with "ORDER BY". How can I speed it up?
My first query was:
SELECT *
FROM wp_usermeta
WHERE meta_key = 'partner'
AND meta_value = 1
ORDER BY user_id DESC
LIMIT 5
It takes 0.2601 seconds. After some research I could optimize it to:
SELECT user_id
FROM wp_usermeta
WHERE meta_key = 'partner'
AND meta_value = '1'
ORDER BY umeta_id DESC
LIMIT 5
This query takes just 0.1491 seconds, but still too much. If I remove the ORDER BY, it takes only 0.0075 seconds.
I read a lot on Stackoverflow and other forums, but I could not get a better output. Has someone an idea?
It is a standard WordPress usermeta table.

Answer 1

The wp_usermeta table in WordPress is well-known and it has a single-column index on meta_key.
But this selects all rows with the specified key, which doesn't narrow down the search much. Also it doesn't help sorting, so the query must do extra work to do the sorting:
mysql> explain SELECT * FROM wp_usermeta WHERE meta_key = 'partner' AND meta_value = 1 ORDER BY user_id DESC LIMIT 5\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: wp_usermeta
type: ref
possible_keys: meta_key
key: meta_key
key_len: 767
ref: const
rows: 1
Extra: Using where; Using filesort
Adding a new index should help:
mysql> alter table wp_usermeta add key (meta_key(191), meta_value(191), user_id);
mysql> explain SELECT * FROM wp_usermeta WHERE meta_key = 'partner' AND meta_value = 1 ORDER BY user_id DESC LIMIT 5\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: wp_usermeta
type: ref
possible_keys: meta_key_2,meta_key
key: meta_key_2
key_len: 767
ref: const
rows: 1
Extra: Using where; Using filesort
Even though this shows it's using the new index (meta_key_2), it's not helping. The key_len and ref indicate it's only using the first column of the index. Why can't it use both columns?
Because your query compares the integer value 1 to a string column meta_value. You must compare similar types, i.e. string '1' to the string column:
mysql> explain SELECT * FROM wp_usermeta WHERE meta_key = 'partner' AND meta_value = '1' ORDER BY user_id DESC LIMIT 5\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: wp_usermeta
type: ref
possible_keys: meta_key_2,meta_key
key: meta_key_2
key_len: 1534
ref: const,const
rows: 1
Extra: Using where
Now it's able to use the second column in the index to search for value '1', you can tell because key_len: 1534 and ref: const,const indicate it's using two columns of the index instead of one column.
Then the optimizer realizes it's already reading the data in order by user_id, so there's no need to sort. The "Using filesort" goes away.

Answer 2

WP has an inefficient schema for its "meta" tables. But they can be fixed. In [here], I discuss several things that need fixing. And I explain the 191 kludge, plus 5 options for avoiding it.
And I don't get into "index merge intersect", since a composite index is always(?) better.