Question

This is the table I have:

CREATE TABLE `person` (
  `id` bigint(10) NOT NULL AUTO_INCREMENT,
  `name` varchar(20) DEFAULT NULL,
  `age` int(10) NOT NULL DEFAULT '0',
  PRIMARY KEY (`id`),
  KEY `age` (`age`)
) ENGINE=InnoDB AUTO_INCREMENT=10000 DEFAULT CHARSET=latin1;

This is the output of explain:

mysql> explain select * from person order by age\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: person
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 10367
        Extra: Using filesort
1 row in set (0.00 sec)

What’s going on? Why isn’t MySQL using the age index to do the sorting? I tried doind analyze table, but it didn’t make any difference.

Just for reference, here’s the distribution of data in the table:

mysql> select age, count(*) from person group by age;
+-----+----------+
| age | count(*) |
+-----+----------+
|  21 |     1250 |
|  22 |     1216 |
|  23 |     1278 |
|  24 |     1262 |
|  25 |     1263 |
|  26 |     1221 |
|  27 |     1239 |
|  28 |     1270 |
+-----+----------+
8 rows in set (0.04 sec)

UPDATE

@grisha seems to think that you can’t select a field not in the index. That doesn’t seem to make any sense, however, it looks like the following works:

mysql> explain select age from person order by age \G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: person
         type: index
possible_keys: NULL
          key: age
      key_len: 4
          ref: NULL
         rows: 10367
        Extra: Using index
1 row in set (0.00 sec)

And also if I add an index that covers all the field it works as well:

mysql> alter table person add key `idx1` (`age`, `id`, `name`);
Query OK, 0 rows affected (0.29 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> explain select * from person order by age\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: person
         type: index
possible_keys: NULL
          key: idx1
      key_len: 35
          ref: NULL
         rows: 10367
        Extra: Using index
1 row in set (0.00 sec)

@eggyal suggested using index hints. This seems to work as well, and is probably the correct answer:

mysql> explain select * from person force key for order by (age) order by age\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: person
         type: index
possible_keys: NULL
          key: age
      key_len: 4
          ref: NULL
         rows: 10367
        Extra: 
1 row in set (0.02 sec)

Answer 1

Index can help you in sorting, when you select only index column. In your case you select *, therefore mysql doesn’t use index.

Why usually index can’t help in sorting ?

If we want to sort some table t by field my_field using index on my_field, we will do :

for each my_field f in index, do :
    get all records where my_field = f and add to result
return result

Assuming not clustered index, the above will execute as many random I/O’s as the number of rows in t(might be huge), whereas simple external sorting algorithm will read the data by blocks/pages sequentially and will execute much less random I/O’s.

So, of course you can say to db : “I want to do sorting using index”, but it’s really not efficient.