Question

• Querycache is disabled!
• Indexes are 100 % loaded in RAM
• index is only ID

1benchmark:

"SELECT title FROM posts LIMIT ?, 15";
? = rand(1,183655);

EXPLAIN 
id  select_type     table   type    possible_keys   key     key_len     ref     rows    Extra
1   SIMPLE  posts   ALL     NULL    NULL    NULL    NULL    183637  

after doing 100 loops it took over 100 sec.

2Benchmark:

"SELECT title FROM posts WHERE id = 78845 OR id = 158738 OR id = 57065 OR id = 146797 OR id = 78918 OR id = 65227 OR id = 117987 OR id = 92541 OR id = 39782 OR id = 1958 OR id = 180384 OR id = 170758 OR id = 102227 OR id = 180223 OR id = 46391";

    in each loop every id is generated via rand(1,183655);

after doing 100 loops it took 6 sec to finish and it uses the primary key.

The problem is I think in the LIMIT MySQL doesn’t use the primary key for the id.

After testing with Limit again:

SECONDS:        LOOPS:  SQL:
41.560034990311 =    30 =   "SELECT title FROM posts LIMIT ?, 15";
36.302664995193 =    30 =   "SELECT title FROM posts ORDER BY id LIMIT ?, 15";
70.335160970688 =    30 =   "SELECT title FROM posts ORDER BY id ASC LIMIT ?, 15";
48.453547000885 =    30 =   "SELECT id, title FROM posts LIMIT ?, 15"

Somehow but I’m not 100 % sure. I updated on my VMware PHP5.1.6 to PHP5.3 since then I noticed, that these LIMIT queries take longer.

Answer 1

You could run this instead:

SELECT title 
FROM posts 
WHERE id >= ?
LIMIT 15

It’s not equivalent to the LIMIT ?, 15 but it will use the index.

You can also check this related article.