DuckDB's CSV Quoting: Why Your Hash Signs Get Extra Quotes

What's up, data enthusiasts! Ever found yourself scratching your head, wondering why your beautifully crafted CSV files exported from DuckDB suddenly have extra, seemingly unnecessary, double quotes around certain fields? Specifically, have you noticed this happening when a simple little character like a hash sign (#) decides to pop up in your data? If so, you're in the right place, because today we're diving deep into a fascinating quirk: DuckDB's CSV minimal quoting rules and how they interact with that unassuming hash symbol when you use the COPY ... TO command. It’s a subtle but important detail that can sometimes throw a wrench in your data pipeline, so let’s get to the bottom of it.

Unpacking the DuckDB Quoting Conundrum: The Hash Sign Mystery

Alright, guys, let’s talk about the bedrock of data interchange for many of us: CSV files. The beauty of CSVs lies in their simplicity and universal readability. But beneath that simple comma-separated facade, there's a standard, a rulebook if you will, that ensures everyone's on the same page. We're talking about RFC 4180, the official specification for Common Format and MIME Type for Comma-Separated Values (CSV) Files. This RFC clearly outlines what makes a CSV file universally parsable, and a big part of that is defining when a field needs to be enclosed in double quotes – this is what we call minimal quoting. According to RFC 4180, a field must be quoted only if it contains a comma, a newline character, or a double quote itself. If a field contains a double quote, that quote then needs to be escaped by doubling it. That's it! Simple, clean, and efficient. Any other character, including spaces, hyphens, or, yes, even a hash sign, should ideally be left unquoted if it doesn't clash with the delimiters or special characters defined by the RFC.

Now, here's where our beloved DuckDB, a fantastic in-process SQL OLAP database, seems to deviate slightly from this minimal quoting philosophy, at least when it comes to the hash sign. When you use the powerful COPY ... TO command in DuckDB to export your data into a CSV file, it appears to apply an extra quoting rule: any field containing a # (hash sign) gets automatically enclosed in double quotes, even if it doesn't contain any of the characters explicitly requiring quoting by RFC 4180. This might seem like a small detail, but in the world of data engineering, especially when dealing with downstream systems that are strict about CSV parsing, or when you're striving for the smallest possible file sizes, these extra quotes can become a nuisance. They increase file size ever so slightly, and more critically, they represent a departure from the widely accepted standard. We expect tools to adhere to standards, especially when they offer an option that implies such adherence. This behavior, while perhaps intended as an extra safety measure by DuckDB developers, contradicts the very definition of minimal quoting, leading to confusion and potential compatibility issues. It's not about whether the file is still readable; it's about whether it conforms to the expected minimal standard. The promise of minimal quoting is to give you the leanest, most standard-compliant output, and when a simple hash symbol forces a deviation, it raises questions about the strictness of that adherence. For many data practitioners, knowing that their exported CSVs are truly minimally quoted according to the RFC is crucial for seamless integration with other tools and systems that expect this exact format. It's an issue of precise control over your data's representation.

Replicating the Hash Sign Quoting Bug: A Step-by-Step Guide

To really understand what's going on, sometimes you just gotta roll up your sleeves and see it for yourself, right, guys? Reproducibility is the cornerstone of identifying and fixing any software quirk, and this DuckDB CSV quoting bug is no exception. Let's walk through the exact steps to demonstrate how DuckDB unexpectedly quotes fields containing a hash sign (#) during a COPY ... TO operation, even when attempting to follow minimal quoting rules. It's super straightforward, and you'll see the unexpected output almost instantly. You'll need a couple of small files and a DuckDB executable – that’s it!

First up, let's create our sample data. We'll use a simple CSV input file that contains a field with a hash sign, but nothing else that should trigger quoting under RFC 4180's minimal rules. We'll call this file testin.csv:

title,format
Elvis 30 #1 Hits,vinyl

Take a good look at that title field: "Elvis 30 #1 Hits". See that lovely little hash sign in there? According to the minimal quoting rules we just discussed, this entire field should not be quoted when exported, because it contains no commas, no newlines, and no double quotes. It's just a regular string with a hash. This is our control, our baseline for what we expect.

Next, we need a SQL script to tell DuckDB what to do. This script will read our input CSV and then attempt to copy it out to another CSV, specifying HEADER and DELIMITER ',' which are standard CSV options. We'll name this test.sql:

COPY (
    SELECT * FROM read_csv_auto('testin.csv', ALL_VARCHAR=TRUE)
) TO 'testout.csv' (HEADER, DELIMITER ',');

A quick note on this SQL: we're using read_csv_auto to automatically infer the schema, and ALL_VARCHAR=TRUE to ensure all columns are treated as strings, which simplifies things and ensures we're focusing purely on the quoting behavior. The COPY ... TO 'testout.csv' (HEADER, DELIMITER ',') command is where the magic (and the mystery!) happens. It's meant to export the data into testout.csv using a comma as a delimiter and including the header row.

Finally, to execute this, if you're on Windows like the original report, a simple batch file test.bat will do the trick:

duckdb.exe -f test.sql

If you're on Linux or macOS, you'd just run duckdb -f test.sql directly in your terminal, assuming duckdb is in your PATH. After running this command, DuckDB will process test.sql and generate testout.csv in the same directory. Now, for the moment of truth. Let's inspect the testout.csv file that DuckDB produced:

title,format