Well, I went and looked at it again and it seems like SQLite databases although larger than my custom format do compress to a 10% smaller size provided there's no indexes. But my custom format uses indexes so it's not really a good comparison. I guess I should see what it's like with indexes.

But at the end of the day SQLite just doesn't meet the needs of my project due to it's size and lack of free decompression support. Passing a decompression stage before giving it to SQLite is unwieldy and requires a separate decompression module and incurs a copy operation. I don't know enough about SQLite to attempt to compile and integrate the zstd decompressor into it.