Showing entries 1 to 6
Displaying posts with tag: Foreign Key (reset)
Hidden Cost of Foreign Key Constraints in MySQL

Do you wonder if MySQL tells you the truth about writes to tables that have foreign key constraints? The situation is complex, and getting visibility on what is really happening can be a problem.

I found this issue intriguing and decided to share and highlight some examples.

Query Plan

Let us take this example table:

CREATE TABLE `product` (
  `category` int NOT NULL,
  `id` int NOT NULL,
  `price` decimal(10,0) DEFAULT NULL,
  PRIMARY KEY (`category`,`id`)

We want to know how costly an example UPDATE against this table will be:

mysql > EXPLAIN update product set id=id+1 where id=65032158 and category=3741760\G
*************************** 1. row ***************************
           id: 1
  select_type: UPDATE
        table: product
   partitions: NULL
         type: range
possible_keys: PRIMARY
          key: PRIMARY
      key_len: 8
          ref: const,const
         rows: 1 …
[Read more]
Dropping the Foreign Key Constraint Using pt-online-schema-change

In this blog post, we’ll look at how to get rid of the unused Foreign Key (FK) constraint and/or related columns/keys with the help of pt-online-schema-change and the power of its plugins.

Before we proceed, here is a useful blog post written by Peter Zaitsev on Hijacking Innodb Foreign Keys.

If you are trying to get rid of an unused foreign key (FK) constraint and related columns from versions older than MySQL 5.6, or tables that cannot be executed with


 because of limitations mentioned here (specifically, …

[Read more]
MySQL Foreign Keys Example & ERROR 1452

So I ran across a situation today dealing with having to update a field but the user was unable to do so because of the related foreign key constraints.

This blog post with be a simple example showing a foreign key and how to update them if you have to do so.

First let us create a simple table and populate it with random data.

CREATE TABLE `table_w_code` (
`SOMECode` varchar(50) COLLATE utf8_unicode_ci NOT NULL,
`NameofCode` varchar(50) COLLATE utf8_unicode_ci NOT NULL,

Now we will need another table that has a foreign key tied to our previous table.

[anothermysqldba]> CREATE TABLE `table_with_fk` (
`SOMEID` varchar(50) COLLATE utf8_unicode_ci NOT NULL,
`SOMECode` varchar(50) COLLATE utf8_unicode_ci NOT NULL,
`Somemorefields` varchar(150) COLLATE utf8_unicode_ci …

[Read more]
MySQL – Alter table-change datatype of column with foreign key

Schema alteration is a big headache especially when it has already reached to production and things get worse when the relations are involved. Below is a short example for altering tables having foreign key constraints. SQL for tables used in example: create database temp;use temp; drop table if exists city; drop table if exists country; […]

The post MySQL – Alter table-change datatype of column with foreign key first appeared on Change Is Inevitable.

Simple Query to identify Foreign Key references on Deadlocked Tables

The other day, I was troubleshooting a deadlock, and I wondered if any of the table’s columns were referenced by any foreign keys (fks) from any other tables in the instance.

Well, this is actually very simple with information_schema (I_S):


Where `parent` is the name of the table you’re searching for.

Note this query does not restrict on the database, or schema, name, but that could easily be added (or any other number of conditions). Here is an example where I only return the most useful columns (which could be useful for determining said conditions):


If …

[Read more]
Advanced InnoDB Deadlock Troubleshooting – What SHOW INNODB STATUS Doesn’t Tell You, and What Diagnostics You Should be Looking At

One common cause for deadlocks when using InnoDB tables is from the existence of foreign key constraints and the shared locks (S-lock) they acquire on referenced rows.

The reason I want to discuss them though is because they are often a bit tricky to diagnose, especially if you are only looking at the SHOW ENGINE INNODB STATUS output (which might be a bit counter-intuitive since one would expect it to contain this info).

Let me show a deadlock error to illustrate (below is from SHOW ENGINE INNODB STATUS\g):

111109 20:10:03
TRANSACTION 65839, ACTIVE 19 sec, OS thread id 4264 starting index read
mysql tables in use 1, locked 1
LOCK WAIT 6 lock struct(s), heap size 1024, 3 row lock(s), undo log entries 1
MySQL thread id 3, query id 74 localhost root Updating
UPDATE parent SET age=age+1 WHERE id=1
[Read more]
Showing entries 1 to 6