A query plan uses loose index scan if “Using index for group-by” appears in the “Extra” column of the EXPLAIN output. In some plans though, “Using index for group-by (scanning)” appears. What does “(scanning)” mean and how is it different from the regular loose index scan?…
Category Archives: Performance
MySQL 8.0: New Lock free, scalable WAL design
The Write Ahead Log (WAL) is one of the most important components of a database. All the changes to data files are logged in the WAL (called the redo log in InnoDB). This allows to postpone the moment when the modified pages are flushed to disk, still protecting from data losses.…
Histogram statistics in MySQL
As of MySQL 8.0.3, you now have the ability to create histogram statistics in order to provide more statistics to the optimizer. In this blog post, we will have a look at how you can create histogram statistics, and we will explain when it might be useful to have histogram statistics.…
MySQL 8.0: Improved performance with CTE
MySQL 8.0 introduces Common Table Expressions (CTE). My colleague Guilhem has written several blog posts on how to use CTEs , and you can also read about it in the MySQL 8.0 Reference Manual. In this blog post, I will focus on how using a CTE instead of a view or a derived table can improve performance.…
MySQL 8.0 Labs: [Recursive] Common Table Expressions in MySQL (CTEs), Part Three – hierarchies
[Update of April 2017: the features discussed here are now also available in the official release 8.0.1 of MySQL. And this series of posts has been continued, here.]
Here is the third in a series of posts about CTEs, a new feature of MySQL 8.0, available in this Labs release.…
MySQL 8.0 Labs – Descending Indexes in MySQL
Starting with the 8.0 optimizer labs release the MySQL server now supports descending indexes. As I will detail in this post, this new feature can be used to eliminate the need for sorting results, and lead to performance improvements in a number of queries.…
MySQL 8.0: Faster batch record retrieval
One of the things we have been working on for MySQL 8, is speeding up scans that read multiple records from a table or an index. Such scans benefit from reading the records in batches, so that they don’t get the overhead of latching data pages and navigating in the B-tree for every single record that is read.…
Indexing JSON documents via Virtual Columns
In a previous post I took the new JSON datatype and functions for a test drive, using sample data provided by SF OpenData. Today I will extend that example to include indexing.
Introducing Generated Columns
MySQL 5.7 introduces a new feature called generated columns.…
What to do with optimizer hints after an upgrade?
MySQL 5.7: Improved Performance of Queries with Derived Tables
In a recent blog post my colleague Roy presented his work for MySQL 5.7 that makes the query optimizer merge sub-queries in the FROM clause (so-called derived tables) with the outer query. I will in this blog post show an example of how this work improves the performance of a MySQL query.…