HSQLDB is a relational database engine written in Java, with a JDBC driver, conforming to ANSI SQL:2023. A small, fast, multithreaded engine and server with memory and disk tables, LOBs, transaction isolation, multiversion concurrency and ACID.
A disk based production quality RTree based upon Guttmans quadratic split algorithm having following features:
Buffering with dirty write,
Index joins,
Tree Seeding,
Window and NN queries,
Runtime STR Packing capability,
Completely Thread safe.
This is an implementation of the java DOM specification that keeps all
its "parsed-node" data in disk files instead of RAM. This means you
can create really big DOM trees an not use all your RAM.