Index

In a recent customer engagement we experienced a lot of corrupted (normal) indexes and also corrupted full-text indexes on InnoDB tables in a Galera Cluster (v10.4.13).

In the error log we did not see which table it was but we have only a few log entries every here and there indicating a full-table index is corrupt:

2020-07-08 22:09:03 0 [ERROR] InnoDB: (Duplicate key) writing word node to FTS auxiliary index table.
2020-07-08 22:09:06 0 [ERROR] InnoDB: (Duplicate key) writing word node to FTS auxiliary …

In the recent MariaDB DBA advanced training class the question came up if MariaDB can make use of an index when searching for NULL values… And to be honest I was not sure any more. So instead of reading boring documentation I did some little tests:

Search for NULL

First I started with a little test data set. Some of you might already know it:

CREATE TABLE null_test (
  id INT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY
, data VARCHAR(32) DEFAULT NULL
, ts TIMESTAMP DEFAULT CURRENT_TIMESTAMP() 
); …

Recently I switched on the MariaDB slow query logging flag log_queries_not_using_indexes just for curiosity on one of our customers systems:

mariadb> SHOW GLOBAL VARIABLES LIKE 'log_quer%';
+-------------------------------+-------+
| Variable_name                 | Value |
+-------------------------------+-------+
| log_queries_not_using_indexes | OFF   |
+-------------------------------+-------+

mariadb> SET GLOBAL log_queries_not_using_indexes = ON;

A tail -f on the MariaDB Slow Query Log caused a …

Table of Contents

• Accounts not properly closing connections
• Unused indexes
• Who created temporary (disk) tables
• Accounts which never connected since last start-up
• Users which never connected since last start-up
• Totally unused accounts (never connected since last restart and not used to check Stored Program or View privileges) since last start-up
• SHOW FULL PROCESSLIST
• Storage Engines per schema
• Tables without a Primary Key
• Bad SQL queries of users
• SHOW PROFILE in PERFORMANCE_SCHEMA
• SELECT, INSERT, UPDATE and …

Recently a customer has shown up with some deadlocks occurring frequently. They were of the following type (I have shortened the output a bit):

*** (1) TRANSACTION:

TRANSACTION 22723019234, fetching rows
mysql tables in use 1, locked 1
LOCK WAIT 7 lock struct(s), heap size 1216, 14 row lock(s)
update location set expires='2012-08-10 04:50:29' where username='12345678901' AND contact='sip:12345678901@192.168.0.191:5060' AND callid='945a20d4-8945dcb5-15511d3e@192.168.0.191'

*** (1) WAITING FOR THIS LOCK TO …

Canias is a great ERP product but there is still some potential to improve it. We only focus on MyISAM/InnoDB installations of Canias v6.0.2 in this article. For other DB back-ends those recommendations might be obsolete.

A general problem is, that in InnoDB Primary Keys are crucial in MySQL. Canias often lacks a Primary Key. In this case InnoDB will use the Unique Key for the Primary Key.

In InnoDB data are sorted by the Primary Key so it is a good idea to have the Primary Key on CLIENT and COMPANY as well …

It is generally well known that indexes help a lot to speed up database queries (especially SELECT but also UPDATE and DELETE). It is less known, that indexes also have some disadvantages.

One of these disadvantages is, that indexes require space in memory and on disk. An other disadvantage of indexes is, that they slow down DML statements like INSERT and DELETE.

We often see at our customers that they do not realized this behavior. Now I found the time to show the impact of (too) many indexes graphically. …

Table of Contents

• Result set with temporary sequence
• Determination of optimal length of prefixed indexes
• Using MySQL keywords in table or columm names
• Missing Primary Key Index
• Problems while installing a MySQL 5.5 database
• InnoDB AUTO_INCREMENT at 2^nd position

Result set with temporary sequence

Sometimes you would like to have a result set with something like a rownum. You can do this at least in the following two ways:

a) with a TEMPORARY MEMORY table:

CREATE TEMPORARY TABLE mem (
    seq  INT NOT NULL …