CREATE TABLE `bookshelf` (
`bookshelf_id` VARCHAR(10) NOT NULL,
`book_name` VARCHAR(512) NOT NULL,
`book_id` VARCHAR(10) NOT NULL,
PRIMARY KEY (`bookshelf_id`, `book_name`(128), `book_id`)
);EXPLAIN SELECT * FROM `bookshelf` ORDER BY `bookshelf_id` LIMIT 5;EXPLAIN SELECT * FROM `bookshelf` ORDER BY `bookshelf_id`, `book_name` LIMIT 5;
In some cases, MySQL cannot use indexes to resolve the ORDER BY, although it may still use indexes to find the rows that match the WHERE clause. Examples:
- There is an index on only a prefix of a column named in the ORDER BY clause. In this case, the index cannot be used to fully resolve the sort order. For example, if only the first 10 bytes of a CHAR(20) column are indexed, the index cannot distinguish values past the 10th byte and a filesort is needed.
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