Comment by ilyagr
17 hours ago
Do the CSV files allow quoted newlines? If yes, what's the trick to avoid checking the whole file too find out whether a newline is quoted or a record separator when reading it from the middle in an async thread?
17 hours ago
Do the CSV files allow quoted newlines? If yes, what's the trick to avoid checking the whole file too find out whether a newline is quoted or a record separator when reading it from the middle in an async thread?
DuckDB uses a speculative parallel CSV parsing technique. The basic idea is that the parser speculates about the state the CSV parser is in at a random byte (e.g., whether it is inside a quoted field) and tries to figure out where the next row starts based on that.There are validation steps during finalization as well, to ensure the parser did not got anything wrong in its speculation.
I've never gotten around to writing a blog post about it, but I go quite in-depth on the technique in this presentation: https://www.youtube.com/watch?v=YrqSp8m7fmk
(Disclaimer: I'm the author of the blog post and also the developer who implemented the entire CSV parser in DuckDB.)
Burning my points to say dude that's sick, parsing CSVs is hell I am genuinely going to watch this thank you.
there isn't any upvote limit on HN.
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Thank you. I love using duckdb and frequently query directly from zstd compressed files (and write out new ones). They’re easy to inspect using visidata and make outer use of. I found the talk interesting and enjoyable.
You’re officially my hero, the duckdb csv parser is the fastest I could find and helped save a data project with hundreds of gigabytes of csv pain.
Thank you so much. I love DuckDB!
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In general, yes. But Hive, for example, doesn't support embedded newlines or record separators that are not newlines, which means that a lot of older CSV files containing big data do not allow quoted newlines.
most likely - they read large chunks and decode them