Select

Translated by deepl.com

Introduction

Two points in advance:

1. A deadlock is a state in which two different transactions are no longer able to continue working because each transaction holds a lock that the other transaction would need. Because both transactions are now waiting for the other transaction to release their locks, neither transaction will release their respective locks. And that would last forever. To avoid this, the MariaDB instance intervenes and kills the transaction that has done less work. …

For processing SELECT queries MySQL needs some times the help of temporary tables. These temporary tables can be created either in memory or on disk.

The number of creations of such temporary tables can be found with the following command:

mysql> SHOW GLOBAL STATUS LIKE 'created_tmp%tables';
+-------------------------+-------+
| Variable_name           | Value |
+-------------------------+-------+
| Created_tmp_disk_tables | 4     |
| Created_tmp_tables      | 36    |
+-------------------------+-------+ …

Its a known problem that changing the Storage Engine from MyISAM to InnoDB can cause some problems
[ 1
] if you have queries of this type:

SELECT COUNT(*) from table;

Luckily this query happens rarely and if, the query can be easily omitted or worked around by guesstimating the amount of rows in the table. For example with:

SHOW TABLE STATUS LIKE 'test';

But in some rare cases customer really needs these values for some reasons. To not exhaust the resources of the server with this query which can be fired …

A customer who is handling digital certificates had a problem with one of those. So we had to investigate.

Because the certificate is in binary form it is stored in a BLOB and we had to extract it from the database to do some verifications.

What first came to my mind was to extract the certificate with the SELECT INTO OUTFILE command. But the verification tool complained and told us that the certificate has a wrong format.

Luckily I found in the documentation the following statement: If you use INTO …