Slow MySQL selects with where PK > x -


i have table 5 million rows of following structure:

create table `books` (   `asin` char(10) character set latin1 not null,   `title` varchar(512) not null, ...other fields...   primary key (`asin`),   key `lang` (`lang`) ) engine=myisam default charset=utf8;

i'm trying run script on all, use "select * books asin > x limit 10000" , remember last pk.

i expected query fast since i'm querying pk, performance degrading query taking 1 minute (used better in beginning). why happen?

explain example:

mysql> explain select * books asin > 'b000ot86oy' limit 10000; +----+-------------+-------+-------+---------------+---------+---------+------+---------+-------------+ | id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows    |       | +----+-------------+-------+-------+---------------+---------+---------+------+---------+-------------+ |  1 | simple      | books | range | primary       | primary | 10      | null | 4670633 | using | +----+-------------+-------+-------+---------------+---------+---------+------+---------+-------------+

i able mitigate issue performing alter table books order asin asc. imagine issue rows physically far apart on disc, after doing above, query became instantaneous.


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