{"id":105217,"date":"2025-02-13T22:56:00","date_gmt":"2025-02-13T22:56:00","guid":{"rendered":"https:\/\/www.red-gate.com\/simple-talk\/?p=105217"},"modified":"2025-03-26T20:20:22","modified_gmt":"2025-03-26T20:20:22","slug":"mysql-index-overviews-descending-b-tree-indexes","status":"publish","type":"post","link":"https:\/\/www.red-gate.com\/simple-talk\/databases\/mysql\/mysql-index-overviews-descending-b-tree-indexes\/","title":{"rendered":"MySQL Index Overviews: Descending B-Tree Indexes"},"content":{"rendered":"\n

Most anyone who works with indexes should be familiar with B-tree indexes. These are the most commonly used data structures to facilitate faster access to data when we search for exact matches of data using the WHERE clause (or partial matches that can take advantage of the sort order of the index). We\u2019ve covered indexes in this blog already<\/a> \u2013 there, we\u2019ve told you that indexes come in a variety of shapes and sizes including B-tree, hash, spatial, prefix, composite, covering, and clustered indexes.<\/p>\n\n\n\n

Some readers of this blog may also know that B-tree indexes have a couple of characteristics unique to themselves: they can also be of an ascending or descending order. In this article, we will look at why you might choose to set an index\u2019 order from the default of ascending for all the key values. Some readers of this blog may also know that B-tree indexes have a couple of characteristics unique to themselves: they can also be of an ascending or descending descent, and this is precisely what this blog is about.<\/p>\n\n\n\n

What are Descending Indexes?<\/h2>\n\n\n\n

Descending indexes are just what they sound like, really \u2013 they are B-tree indexes that store rows in a descending order based on the index key configuration. The same can be said about ascending indexes, too \u2013 such indexes work the same way, just the other way around: they store rows in an ascending order.<\/p>\n\n\n\n

Such behavior of indexes becomes very useful when our use case necessitates the use of the ORDER BY<\/code> clause. Think about it \u2013 the ORDER BY clause \u201cgives an order\u201d to our database so that the database sorts the data in a certain way, In other words, the order of rows becomes important once we find ourselves running queries with the ORDER BY clause \u2013 the clause comes after our SELECT definition and looks like so:<\/p>\n\n\n\n

SELECT `col1`, `col2` 
FROM `demo_table` \nORDER BY `id` ASC|DESC;<\/pre>\n\n\n\n
Such queries are nothing fancy, and indexes in action when such operations are being run don\u2019t have anything fancy within themselves either: queries will be constructed with the ORDER BY<\/code> and ASC<\/code> or DESC<\/code> clauses (example above), while indexes will have only the ASC<\/code> or DESC<\/code> clauses (examples are provided below.)<\/p>\n\n\n\n
Both ascending and descending index keys are supported in many database management systems, including MySQL and all of its flavors. As such, descending indexes have limitations and conditions that mark lines they aren\u2019t allowed to cross:<\/p>\n\n\n
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