Module Sqlite_zstd

Compression mode requested at registration time. Captured per connection in the C extension's user-data slot, so the 1-arg form of zstd_compress resolves it without any SQL-level binding. Level n with n <= 0 is treated as No_compression (the 1-arg form passes the input through unchanged).

Register user-defined SQLite functions on the given connection:

• zstd_compress(blob) (1-arg): compresses blob using the compression mode chosen at registration time. No_compression returns blob unchanged (storage class preserved); Level n produces a zstd frame at level n.
• zstd_compress(blob, level INTEGER) (2-arg): compresses blob using the explicit level, independent of registration mode. level <= 0 is a pass-through.
• zstd_decompress(blob): inverse of zstd_compress. If the input does not start with the zstd magic number (0x28B52FFD little-endian), the input is returned unchanged — this backward-compatibility property lets the same query work on both compressed and uncompressed rows.

zstd_compress additionally passes inputs of 16 bytes or fewer through unchanged: the minimum zstd frame envelope (magic + frame header + block header + optional checksum) is around 12-16 bytes, so any payload up to that threshold is guaranteed to grow when compressed. Pass-through keeps storage tight on tiny values and round-trips through zstd_decompress.