为什么Derby数据库占用这么多空间？

Question

I am new to databases and I love how easy it is to get data from a relational database (such as a Derby database). What I don't like is how much data one takes up; I have made a database with two tables, written a total of 130 records to these tables (each table has 6 columns), and the whole relational database gets saved in the system directory as a folder that houses a total of approximately 1914014 bytes! (Assuming that I did the arithmetic right....) What the heck is going on to cause such a huge request of memory?! //I also notice that there is a log1.dat file in log folder that takes up exactly 1MB of data. I looked into this file via Notepad++, and saw that it was mostly NULL characters. What is that all about?

Answer 1

Derby need to keep track on your database data, the redo logs and transactions so your database is in a consistent state and can recover even from pc crashes. Also he creates most files with a fixed size (like 1MB) to ensure he did not need to increase the file size later on (performance issues and to not fragment his files to much).

Over the runtime or when stopping, Derby will clean up some of this files or regroup them and free space.

So overall the space and the files are the trade offs you get for using a database.

Maybe you can change some of this behaviour via some Derby configs (I did not find any one suitable in the doc :().

Answer 2

When last checked in 2011, an empty Derby database takes about 770 K of disk space: http://apache-database.10148.n7.nabble.com/Database-size-larger-than-expected-td104630.html

The log1.dat file is your transaction log, and records database changes so that the database can be recovered if there is a crash (or if your program calls ROLLBACK).

Note that log1.dat is disk space , not memory .

If you'd like to learn about the basics of Derby's transaction log, start here: http://db.apache.org/derby/papers/recovery.html