Postgres 19 Compression: from pglz to LZ4

Summary

Postgres 19 is planning to change the default TOAST compression from pglz to LZ4, so let's look at how Postgres compresses data in table storage and indexes. Postgres uses a single, unified compression framework for table (heap), TOAST, and indexes. In heap and TOAST, compression is automatic and on by default for variable-length types like TEXT, VARCHAR, BYTEA, and JSONB. In indexes, it is opportunistic: compression fires only when an individual key exceeds the size threshold, not for every variable-length value stored in an index. click to expandCompression was first added in Postgres 7.0, released in 2000, but not in the form it takes today. At the time, Postgres had a strict 8kB maximum row size, and trying to insert more than 8kB would throw an error. Fixing this row size limit was a priority for the Postgres core team.The first attempt to work around the limit was an explicitly compressed field. Postgres 7.0 shipped an lztext data type that used the pglz compression algorithm. This implementation had tradeoffs: the 8kB row limit still existed, and users had to explicitly choose a compressed data type.After the 7.0 release, the next logical step might have been to add more compressed data types like LONG or BLOB (as many closed source databases were doing at the time). Instead, the Postgres core team rejected additional data types and rallied around TOAST. Discussions on the pgsql-hackers mailing list show the group split the 8kB problem into two distinct problems: a data type problem and a physical storage problem. Compression and TOAST were the answer to the physical storage problem.With Postgres 7.1, TOAST was implemented using pglz.The pglz algorithm lives in the pg_lzcompress.c file. Because Postgres is open source, we can read the author's reasoning for home-rolling a compression algorithm right in the comments:Trade ratio for speed: pglz is fast to compress and decompress, and willingly gives up compression ratio to get there. The source code calls this ...