Tagged with mysql

tekx – lig’s talk on scalability and mysql

@lig will be talking about mysql 5.5 and scalability this session She is Senior Technical Support Engineer for MySQL.

We will be covering

  • semi-synchronous replication
  • performance schema
  • SIGNAL/RESIGNAL
  • more partitioning options
  • InnoDB – LOTS of InnoDB (performance and scalability improvements)

In 5.5 InnoDB will be the default!!! WOOT.

Default replication is asynchronous. Meaning master writes to binary log and the slave connects and “pulls” contents of the binary log. Bad thing is if the master crashes, there’s no guarantee that a slave has all committed transanction.

Simi-Synchronous Replication is an alternative to asynchronous replication. Midway point between asynchronous and fully syncronous. Master only waits for a slave to receive an event. Don’t have to wait for slaves to actually commit.

Performance schema tracks at an extremely low level. Just like Information schema, tables are views or temporary tables. Activation doesn’t cause any change in server behavior. This is designed for advanced users.

Think of SIGNAL as an exception, a way to “return” an error. You get exception-handling logic for stored procedures, stored functions, triggers,events and db apps.

RESIGNAL lets you pass error information up. Think of it as a catch. Requres an active handler to execute. Lets you program on your PHP side to catch that very specific handling.

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mysql alter table add foreign key

I recently ran into a situation where I needed to add a foreign key to a table. This seems easy at first but was actually a 3 step process compared to the couple of lines it takes in a create table sytax.

Short version: when creating your FK column, it helps to have the column definition of the FK match the column definition. In retrospec, this seems like a no-brainer but was the heart of my problem.

So, you want create a FK to an existing table. Lets start by creating our table.

CREATE TABLE IF NOT EXISTS Role(
Id INT(11) UNSIGNED NOT NULL AUTO_INCREMENT,
Name VARCHAR(255) NOT NULL UNIQUE,
Description TEXT DEFAULT NULL,
PRIMARY KEY(Id)
);

Pretend we pooped data into it and now it’s a big pain in the ass to do a drop/create with how we want things.

First we add our column that will be the Foreign Key.

ALTER TABLE Role ADD COLUMN Parent int(11) UNSIGNED DEFAULT NULL;

Next we add an index. Why? because mysql says so and FKs rely on indexes

ALTER TABLE Role ADD INDEX Parent (Parent);

Now we add our constraint.

ALTER TABLE `Role` ADD CONSTRAINT `Parent` FOREIGN KEY(`Parent`) REFERENCES `Role`(`Id`) ON DELETE SET NULL ON UPDATE SET NULL;

For fun, do

ALTER TABLE Role ADD COLUMN Parent int(11) DEFAULT NULL;

for the first step. Everything will look awesome until you get to the the last step. You’ll get a wonderful “Errno 150” error and spend a bunch of time googling various key words trying to figure out what went wrong. Well, at least I did.