> Wrong. SQL databases (except for SQLite) are almost completely interchangeable because they are all based on the same relational model and they all implement

> Wrong. SQL databases (except for SQLite) are almost completely interchangeable because they are all based on the same relational model and they all implement the ANSI SQL standard with only minor deviations. Wrong again. Here's a very trivial example. To index or not to index? On MySQL InnoDB you usually get a major performance benefit out of indexing just about everything you want to search on later. On PostgreSQL you usually get a major performance benefit out of indexing columns only as needed later. This is because of internal design differences, sequential scans through a table are much cheaper on PostgreSQL (in MySQL it has to scan a table in key order, not physical order which means no OS prefetch). Also non-PK index scans are faster on PostgreSQL though PK index scans are slightly faster on InnoDB (basically the table is contained in the Primary Key index there). Contrast the fairly easy and routine process of migrating between MySQL, PostgreSQL, or SQL Server with the huge amount of work involved changing your application code from MongoDB to anything else. So suppose I want to move my database which uses PostGIS and pgcrypto to MySQL or SQL Server. How would I do that?