Tuesday, October 24, 2006

Some Interesting MySQL facts

I've been doing a lot of research into the internals of MySQL in order to develop DrySQL, and here are some useful bits of information that I came across:

1) Check constraints are not supported in the latest version of MySQL (5.1.x). The syntax for check constraints is supported, but any check constraints in your alter/create table syntax are just ignored.
Official MySQL documentation.

2) The REFERENTIAL_CONSTRAINTS table was added to the INFORMATION_SCHEMA database as of MySQL 5.1.10.
Bug Report/Feature Request
Official MySQL documentation

This is in compliance with the ISO SQL:2003 standard. This table is useful to those who want information about the "ON UPDATE" and "ON DELETE" rules associated with a table constraint. Currently, the only solution for retrieving this info is to parse the SHOW CREATE TABLE output, which is a pretty ugly hack and a performance hit compared to retrieving the constraint info directly from the INFORMATION_SCHEMA DB.

3) MySQL creates implicit default values for columns if no default is specified. If the column is nullable, then MySQL sets the default value to NULL. If the column is not nullable, the default is value is set the MySQL's implicit default value for that column type (check this link for details about implicit defaults).

Suppose I create the following table:


If I query the information schema's COLUMNS table to find out whether a default value has been specified for the ID column in the TEST table, MySQL will return to me the empty string as the default value. Does this mean that the empty string was defined as the default value for this column, or that no default was specified at all? Unfortunately, there is no way to make this determination in the latest version of MySQL, which is a pain for those of us developing DB tools against MySQL.

Errata: As of version 5.0.2, MySQL no longer creates implicit default values when no default is specified

4) Roland Bouman created an excellent clickable diagram of the MySQL Information Schema, which can be found here.

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