October 21, 2014

Multiple column index vs multiple indexes

(There is an updated version of the content in this post by Percona’s Stephane Combaudon available here.) After my previous post there were questions raised about Index Merge on Multiple Indexes vs Two Column Index efficiency. I mentioned in most cases when query can use both of the ways using multiple column index would be […]

Sphinx 0.9.8 is released just in time for OSCON 2008

As you probably already seen in a post by Baron, Sphinx Release 0.9.8 is finally out, just in time for OSCON 2008. Even though it is “minor release” if you look at the number, it is major release in practice (and you can view snapshots as minor releases). The changes since 0.9.7 are dramatic with […]

Redundant index is not always bad

About year ago Peter wrote about redundant indexes and mentioned sometimes it is good to leave two indexes, even one is first part of another. I’m speaking about BTREE indexes, for example, KEY (A), and KEY (A,B). From SQL point of view KEY(A) is not needed, as for queries like WHERE A=5 the index (A,B) […]

Using index for ORDER BY vs restricting number of rows.

One interesting problem with MySQL Optimizer I frequently run into is making poor decision when it comes to choosing between using index for ORDER BY or using index for restriction. Consider we’re running web site which sell goods, goods may be from different categories, different sellers different locations which can be filtered on, and there […]

INSERT ON DUPLICATE KEY UPDATE and REPLACE INTO

Jonathan Haddad writes about REPLACE INTO and INSERT ON DUPLICATE KEY UPDATE. Really, Why MySQL has both of these, especially both are non ANSI SQL extensions ? The story here seems to be the following – REPLACE INTO existed forever, at least since MySQL 3.22 and was a way to do replace faster and what […]

Duplicate indexes and redundant indexes

About every second application I look at has some tables which have redundant or duplicate indexes so its the time to speak about these a bit. So what is duplicate index ? This is when table has multiple indexes defined on the same columns. Sometimes it is indexes with different names, sometimes it is different […]

Using UNION to implement loose index scan in MySQL

One little known fact about MySQL Indexing, however very important for successfull MySQL Performance Optimization is understanding when exactly MySQL is going to use index and how it is going to do them. So if you have table people with KEY(age,zip) and you will run query something like SELECT name FROM people WHERE age BETWEEN […]

Descending indexing and loose index scan

Comments to my previous posts, especially this one by Gokhan inspired me to write a bit about descending indexes and about loose index scan, or what Gokhan calls “better range” support. None of these are actially related to Innodb tables in general – these are features MySQL should get for all storage engines at some […]

Innodb transaction history often hides dangerous ‘debt’

In many write-intensive workloads Innodb/XtraDB storage engines you may see hidden and dangerous “debt” being accumulated – unpurged transaction “history” which if not kept in check over time will cause serve performance regression or will take all free space and cause an outage. Let’s talk about where it comes from and what can you do […]

MySQL compression: Compressed and Uncompressed data size

MySQL has information_schema.tables that contain information such as “data_length” or “avg_row_length.” Documentation on this table however is quite poor, making an assumption that those fields are self explanatory – they are not when it comes to tables that employ compression. And this is where inconsistency is born. Lets take a look at the same table […]