I am using PostgreSQL from a Kotlin (JVM) application with the pgJDBC driver. According to the documentation, you can get results from a query based on a cursor to avoid loading the whole result set into the application’s memory at once by calling the setFetchSize() method with a positive value on the Statement before issuing the query. I had a non-trivial query which generated a lot of rows (several thousands), and I only needed the first hundred or so.

I have written about using an embedded database like HSQLDB or H2 for testing Java applications. This can still be useful (although JavaEE is not so popular any longer). But even though H2 claims to emulate PostgreSQL and several other popular production SQL databases, sometimes your application uses very specific features of a particular database and you need to test against the real thing to be really sure your application works correctly.

The MySQL database can store large chunks of binary data (up to 1 GB) using the BLOB data types. However, this does not work well if you access the MySQL database from Java (or any other JVM based language) using the MySQL JDBC driver. The JDBC API supports an efficient stream based way to handle BLOBs, but the MySQL JDBC driver does not implement this properly. It works, but it will buffer all data in memory in a way which is very inefficient and can make your JVM run out of memory if the BLOBs are large.