Efficiently compressing string columnar data using frequent pattern mining

摘要

In modern column-oriented databases, compression is important for improving I/O throughput and overall database performance. Many string columnar data cannot be compressed by special-purpose algorithms such as run-length encoding or dictionary compression, and the typical choice for them is the LZ77-based compression algorithms such as GZIP or Snappy. These algorithms treat data as a byte block and do not exploit the columnar nature of the data. In this thesis, we develop a compression algorithm using frequent string patterns directly mined from a sample of a string column. The patterns are used as the dictionary phrases for compression. We discuss some interesting properties of frequent patterns in the context of compression, and develop a pruning method to address the cache inefficiencies in indexing the patterns. Experiments show that our compression algorithm outperforms Snappy in compression ratio while retains compression and decompression speed.