const ZSTD_LEVEL: i32 = 3;Expand description
The ZSTD level every compressed table is written at.
The same as arrow-ipc’s default, and set explicitly anyway: the ratio in
docs/architecture.md section 11 is a published number, and an upstream
default that moved would move it without anything in this tree changing.
3 and not higher, which was measured and rejected. Over 8 real blocks per
signal, level 9 buys 2.4% fewer bytes (8.41x against 8.21x) and takes
1.4x as long to rewrite a block. Compaction is a background sweep sharing
cores with ingest, and a sweep that falls behind leaves blocks uncompressed,
so 2.4% does not buy the risk. Nearly all of the ratio comes from the
dictionary column in crate::schema::ATTRS, not from the level.