mariadb ignoring force index I'm trying to tune a relatively simple query:

select bt.col1,bt.col2
from bigtable bt
join smalltable st on bt.smalltable_id = st.smalltable_id
where st.name = 'some name occuring only once in st'
limit 10


The number of matches in bigtable is relatively small compared to the overall size (&lt; 1%)

Here's the explain plan:

+------+-------------+-------+--------+-----------------+---------+---------+-----------------+----------+-------------+
| id   | select_type | table | type   | possible_keys   | key     | key_len | ref             | rows     | Extra       |
+------+-------------+-------+--------+-----------------+---------+---------+-----------------+----------+-------------+
|    1 | PRIMARY     | bt    | ALL    | ix_smalltable_id| NULL    | NULL    | NULL            | 22709766 | Using where |
|    1 | PRIMARY     | st    | eq_ref | PRIMARY,ix_name | PRIMARY | 2       | bt.smalltable_id|        1 | Using where |
+------+-------------+-------+--------+-----------------+---------+---------+-----------------+----------+-------------+


Somehow, even though it would be easier to pickup the index, it doesn't.

I then tried to force the index:

select bt.col1,bt.col2
from bigtable bt force index (ix_smalltable_id)
join smalltable st on bt.smalltable_id = st.smalltable_id
where st.name = 'some name occuring only once in st'
limit 10


But the query plan and query time is the same. It doesn't want to use the index.

I tried doing where bt.smalltable_id in (select ...), but same query plan and time.

But if I fetch the smalltable_id first, and then embed it in the select, it's much faster.

The question

Can I force the index by enabling some flags? Is this a limitation of the query engine?

Table structure

CREATE TABLE `smalltable` (
  `smalltable_id` smallint(5) unsigned NOT NULL AUTO_INCREMENT,
  `name` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`smalltable_id`),
  KEY `ix_name` (`name`(10))
) ENGINE=InnoDB AUTO_INCREMENT=5698 DEFAULT CHARSET=utf8

CREATE TABLE `bigtable` (
  `bigtable_id` bigint unsigned NOT NULL AUTO_INCREMENT,
  `col1` varchar(255) DEFAULT NULL,
  `col2` varchar(255) DEFAULT NULL,
  ...
  `smalltable_id` smallint(5) unsigned DEFAULT NULL,
  PRIMARY KEY (`bigtable_id`),
  KEY `ix_smalltable_id` (`smalltable_id`)
) ENGINE=InnoDB AUTO_INCREMENT=23167374 DEFAULT CHARSET=utf8
I'm pretty sure the problem is the "partial" index:

CREATE TABLE `smalltable` (
  ...
  KEY `ix_name` (`name`(10))
) 


Try running the query after adding an index on the full length of the column:

ALTER TABLE smalltable
    DROP INDEX ix_name
  , ADD INDEX ix_name_full (name) ;