Assume we have the following table that will contain information about the company employees.

It is clear from the below result that the Backward Scan method is the optimal scanning method to get the maximum ID value: As I mentioned at the beginning of this article, query indexing is the most important key in the performance tuning and optimization process. The primary limitation of not being able to support descending indexes is that the optimizer mus…

Opinions expressed by DZone contributors are their own. Can a MySQL 5.6 master replicate to a 5.7 slave, which in turn replicates to a MySQL 8 slave? Ahmad Yaseen is a SQL Server database administration leader at Aramex International Company with a bachelor’s degree in computer engineering as well as .NET development experience. In SQL Server, the indexes (both, clustered and nonclustered) are created using a B-tree structure, in which each page acts as a doubly-linked list node, having information about the previous and the next pages. Mathematica integrates too well using the "code" I wrote. The author, Chaithra Gopalareddy, of the related article, MySQL 8.0 Labs – Descending Indexes in MySQL, explains in a comment why backwards index scans are slightly less efficient than forward scans: Thanks for showing interest in the new feature. This is a similar situation to an index on (event_date, name) sorted in ascending order, and can be used to satisfy both event_date asc, name asc and event_date desc, name desc (same order across two fields).. The Overflow #47: How to lead with clarity and empathy in the remote world, Feature Preview: New Review Suspensions Mod UX, 2020 Community Moderator Election Results, Slow queries with many self joins in variable-column database, Using an index for both asc and desc on a string column, 'SELECT' slower than 'SELECT my_table.

Empirical tests show that a query like this on an InnoDB table: is faster than its counterpart with ORDER BY ... DESC. For example in MySQL 5.7: While it should be noted that the MySQL 5.7 optimizer is able to scan an ascending index backwards (to give descending order), it comes at a higher cost. So this has nothing to do with ascending indexes in 8.0. In this blog we'll discuss descending indexes in MySQL 8.0. The main drawback of the Backward scan method is that the SQL Server Query Optimizer will always choose to execute it using serial plan execution, without being able to take benefits from the parallel execution plans. Here is how our query from the above performs: I’ve created an index, targeted for our specific query order: event_date descending, name ascending. This is a similar situation to an index on (event_date, name) sorted in ascending order, and can be used to satisfy both event_date asc, name asc and event_date desc, name desc (same order across two fields). As shown further down, we can see forward index scans are ~15% better than backward index scans. At the same time, MySQL (all versions) can scan the index backward, so those two queries will use index: In the second query, MySQL scans the index backward for two fields. Not sure why a covering index is not used, however you can see that backward index scan is slower: A comment (by the author) in the linked article explains: Why scanning an index backwards is slower?

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MySQL scans the table in the order of index (and avoids filesort), then it aborts the scan after finding 10 rows. .sp-force-hide { display: none;}.sp-form[sp-id="159575"] { display: block; background: #ffffff; padding: 15px; width: 420px; max-width: 100%; border-radius: 8px; -moz-border-radius: 8px; -webkit-border-radius: 8px; border-color: #dddddd; border-style: solid; border-width: 1px; font-family: "Segoe UI", Segoe, "Avenir Next", "Open Sans", sans-serif; background-repeat: no-repeat; background-position: center; background-size: auto;}.sp-form[sp-id="159575"] input[type="checkbox"] { display: inline-block; opacity: 1; visibility: visible;}.sp-form[sp-id="159575"] .sp-form-fields-wrapper { margin: 0 auto; width: 390px;}.sp-form[sp-id="159575"] .sp-form-control { background: #ffffff; border-color: #cccccc; border-style: solid; border-width: 1px; font-size: 15px; padding-left: 8.75px; padding-right: 8.75px; border-radius: 6px; -moz-border-radius: 6px; -webkit-border-radius: 6px; height: 35px; width: 100%;}.sp-form[sp-id="159575"] .sp-field label { color: #444444; font-size: 13px; font-style: normal; font-weight: bold;}.sp-form[sp-id="159575"] .sp-button-messengers { border-radius: 6px; -moz-border-radius: 6px; -webkit-border-radius: 6px;}.sp-form[sp-id="159575"] .sp-button { border-radius: 4px; -moz-border-radius: 4px; -webkit-border-radius: 4px; background-color: #da4453; color: #ffffff; width: auto; font-weight: bold; font-style: normal; font-family: "Segoe UI", Segoe, "Avenir Next", "Open Sans", sans-serif; box-shadow: inset 0 -2px 0 0 #bc2534; -moz-box-shadow: inset 0 -2px 0 0 #bc2534; -webkit-box-shadow: inset 0 -2px 0 0 #bc2534;}.sp-form[sp-id="159575"] .sp-button-container { text-align: center;}. Arithmetic on varchars (for "amounts", etc) will be messy. Although the forward scan is the default and heavily known index scanning method, SQL Server provides us with the ability to scan the index rows within the B-tree structure from the end to the beginning. Does a bronze dragon's wing attack work underwater? These values show us how smart the SQL Server Engine is when choosing the most suitable and fastest index scanning direction to retrieve data for the user, which is Forward Scan in the first case and Backward Scan in the second case, as clear from the statistics below: Let us visit the previous MAX example again. By using our site, you acknowledge that you have read and understand our Cookie Policy, Privacy Policy, and our Terms of Service. Our query will look like this: “select * from events order by event_date desc, profile_id asc limit 10”. Podcast 286: If you could fix any software, what would you change? Although the forward scan is the default and heavily known index scanning method, SQL Server provides us with the ability to scan the index rows within the B-tree structure from the end to the beginning. handy SSMS add-in for analyzing the status of SQL indexes and fixing issues with index fragmentation. In the previous query, if we arrange to add a non-clustered index on the EmploymentDate column of the CompanyEmployees table, using the CREATE INDEX T-SQL statement below: Checking the execution plans generated after execution of the two queries, you will see that a seek will be performed on the newly created nonclustered index, and both queries will scan the index from the beginning to the end using the Forward Scan method, without the need to perform a Backward Scan to speed up the data retrieval, although we used the DESC sorting option in the second query.

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