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===compare===
:The current versions are MySQL 5.1 and PostgreSQL 8.4.
#PostgreSQL is a unified database server with a single storage engine. MySQL has two layers, an upper SQL layer and a set of storage engines. [http://www.wikivs.com/wiki/MySQL_vs_PostgreSQL]
#MySQL began development with a focus on speed while PostgreSQL began development with a focus on features and standards. Thus, MySQL was often regarded as the faster of the two.
# Both PostgreSQL and MySQL support Not-Null, Unique, Primary Key and Foreign Key constraints. However MySQL silently ignores the CHECK constraint
#MySQL supports stored procedures, per se; PostgreSQL supports stored functions, which are in practice very similar.
# Trigger - Both PostgreSQL and MySQL support triggers. A PostgreSQL trigger can execute any user-defined function from any of its procedural languages, not just PL/pgsql.MySQL triggers are activated by SQL statements only. They are not activated by changes in tables made by APIs that do not transmit SQL statements to the MySQL Server; in particular, they are not activated by updates made using the NDB API. PostgreSQL has always been strict about making sure data is valid before allowing it into the database, and there is no way for a client to bypass those checks.
# DataTypes - PostgreSQL does not have an unsigned integer data type, but it has a much richer data type support in several aspects: standards compliance, the logically fundamental data type BOOLEAN, user-defined data types mechanism, built-in and contributed data types. PostgreSQL allows columns of a table to be defined as variable-length multidimensional arrays. Arrays of any built-in or user-defined base type, enum type, or composite type can be created. Arrays of domains are not yet supported. MySQL does not have network IP address data types that PostgreSQL has but does provide INET_ATON() and INET_NTOA() functions to convert IPv4 addresses to and from easily stored integers. Postgres does very good job supporting referential integrity, has transactions and rollbacks, foreign keys ON DELETE CASCADE and ON UPDATE CASCADE.
#Security - MySQL has exceptionally good fine-grained access control. You can GRANT and REVOKE whatever rights you want, based on user name, table name and client host name.
#Alter table - Postgres supports ALTER TABLE to some extent. You can ADD COLUMN, RENAME COLUMN and RENAME TABLE. MySQL has all options in ALTER TABLE - you can ADD column, DROP it, RENAME or CHANGE its type on the fly - very good feature for busy servers, when you don't want to lock the entire database to dump it, change definition and reload it back.
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: Compare SQL Implemenations[http://troels.arvin.dk/db/rdbms/ ]
: Compare Postgre and MySQL [http://www-css.fnal.gov/dsg/external/freeware/pgsql-vs-mysql.html]