Performance. Users/testers of the two databases say that when the data model is complicated enough, PostgreSQL is faster than MySQL. Indeed, updates in PostgreSQL 7.4 included an impressive list of performance enhancements. To wit: Several major performance enhancements have been added in Version 7.4, enabling PostgreSQL to match or exceed the speed of other enterprise database systems. These included:
Hash aggregation in memory to make data warehousing and OLAP queries up to 20 times faster;
Improvements in subquery handling by the planner resulting in up to 400 percent speed increases in some complex queries;
New script to set more reasonable postgresql.conf defaults for shared buffers, yielding better "out of the box" performance;
New wire protocol (Version 3) increases the speed of data transfers;
Enhanced implementation of functional indexes allows better indexing on custom data types and composite fields.
Next page: Which database has better support?
On the other hand, MySQL proponents say that, if configured properly, MySQLs MyISAM tables are indeed lightweight and make for a faster database.