Question

For the same data set, with mostly text data, how do the data (table + index) size of Postgresql compared to that of MySQL?

• Postgresql uses MVCC, that would suggest its data size would be bigger

• In this presentation, the largest blog site in Japan talked about their migration from Postgresql to MySQL. One of their reasons for moving away from Postgresql was that data size in Postgresql was too large (p. 41):
  Migrating from PostgreSQL to MySQL at Cocolog, Japan’s Largest Blog Community

• Postgresql has data compression, so that should make the data size smaller. But MySQL Plugin also has compression.

Does anyone have any actual experience about how the data sizes of Postgresql & MySQL compare to each other?

Answer 1

• MySQL uses MVCC as well, just check
  innoDB. But, in PostgreSQL you can
  change the FILLFACTOR to make space
  for future updates. With this, you
  can create a database that has space
  for current data but also for some
  future updates and deletes. When
  autovacuum and HOT do their things
  right, the size of your database can
  be stable.
• The blog is about old versions, a lot
  of things have changed and PostgreSQL
  does a much better job in compression
  as it did in the old days.
• Compression depends on the datatype,
  configuration and speed as well. You
  have to test to see how it’s working
  for you situation.

I did a couple of conversions from MySQL to PostgreSQL and in all these cases, PostgreSQL was about 10% smaller (MySQL 5.0 => PostgreSQL 8.3 and 8.4). This 10% was used to change the fillfactor on the most updated tables, these were set to a fillfactor 60 to 70. Speed was much better (no more problems with over 20 concurrent users) and data size was stable as well, no MVCC going out of control or vacuum to far behind.

MySQL and PostgreSQL are two different beasts, PostgreSQL is all about reliability where MySQL is populair.