MySQL stores active table descriptors in a special memory buffer called the table open cache. This buffer is controlled by configuration variables table_open_cache that hold the maximum number of table descriptors that MySQL should store in the cache, and table_open_cache_instances that stores the number of the table cache instances. With default values of table_open_cache=4000 and table_open_cache_instances=16, MySQL will create 16 independent memory buffers that will store 250 table descriptors each. These table cache instances could be accessed concurrently, allowing DML to use cached table descriptors without locking each other.

If you use only tables, the table cache does not require a lot of memory because descriptors are lightweight, and even if you significantly increase the value of the table_open_cache, the required memory amount would not be so high. For example, …

MySQL stored procedures, functions and triggers are tempting constructs for application developers. However, as I discovered, there can be an impact on database performance when using MySQL stored routines. Not being entirely sure of what I was seeing during a customer visit, I set out to create some simple tests to measure the impact of triggers on database performance. The outcome might surprise you.

Why stored routines are not optimal performance wise: short version

Recently, I worked with a customer to profile the performance of triggers and stored routines. What I’ve learned about stored routines: “dead” code (the code in a branch which will never run) can still significantly slow down the response time of a function/procedure/trigger. We will need to be careful to clean up what we do not need.

Profiling MySQL stored functions

Let’s compare these four simple stored functions (in MySQL 5.7): …

In this post we’ll review how MySQL triggers can affect queries.

Contrary to what the documentation states, we can activate triggers even while operating on views:

https://dev.mysql.com/doc/refman/5.7/en/triggers.html

Important: MySQL triggers activate only for changes made to tables by SQL statements. They do not activate for changes in views, nor by changes to tables made by APIs that do not transmit SQL statements to the MySQL server.

Be on the lookout if you use and depend on triggers, since it’s not the case for updatable views! We have reported a documentation bug for this but figured it wouldn’t hurt to mention this as a short blog post, too. The link to the bug in question is here:

I recently had an opportunity to migrate a customer from a physical server into Amazon’s RDS environment. In this particular case the customers’ platform makes extensive use of MySQL triggers and views.  I came across two significant issues that prevented me from following Amazon’s documentation, which basically states “use mysqldump” but doesn’t call out a specific method of dealing with MySQL triggers and views.

Amazon Relational Database Service (Amazon RDS) is a great platform if you’re looking for complete hands-off management of your MySQL environment, but comes at a cost in the area of flexibility, i.e. you don’t have SUPER privilege and this brings up additional challenges.

1. You need to ensure you set log_bin_trust_function_creators=1 ( by default this is off, 0).
2. You need to clean up your mysqldump syntax.

#1 is easy, you simply make a configuration change …

One of the students wanted an equivalent example to an Oracle DML trigger sample that replaces a white space in a last name with a dash for an INSERT statement. Apparently, the MySQL trigger example in the Oracle Database 11g and MySQL 5.6 Developer Handbook was a bit long. I have to agree with that because the MySQL DML trigger demonstrated cursors and loops in the trigger code.

Triggers can be statement- or row-level actions. Although some databases let you define statement-level triggers, MySQL doesn’t support them. MySQL only supports row-level triggers. Row-level triggers support critical or non-critical behaviors. Critical behavior means the trigger observes an insert, update, or delete that must be stopped, which means it …

Here is a sample table you can create to test following problem/solution on: CREATE TABLE `t1` ( `a` char(1) default NULL, `b` smallint(6) default NULL ); insert into t1 values ('y','1'); I have a table t1 which has column a and b, I want column a to be updated to ‘n’ when column b = 0. Here is the first version [...]