Simon Willison’s Weblog

I was out walking the dog when I got curious about how easy/difficult it would be to write a SQLite C extension that adds a parse_number() function for turning strings like -$1,234.56 or '12.5%' into numeric values that could be summed and sorted.

I fired up regular Claude chat (Opus 5.5) and prompted:

I'm thinking about a SQLite custom function 'parse_number(value)' that strips ',', '$', '£' and '%', handles a leading '-', and returns 'NULL' for anything it can’t parse

Explore the difference between building this as a SQLite extension in C v.s. in Rust - I want to compare performance and code complexity and size of the resulting.so(or whatever) file

This gave me a solid starting point - the Rust extension was 321 KB (using Rusqlite) and 298 KB (using sqlite-loadable-rs), while the C extension was 14 KB. But the Rust extension was faster!

Turns out that's because the Rust equivalent of C's strtod is "twice as fast". So I prompted:

Try writing your own C function for parsing text into a decimal number that catches up to or beats the Rust implementation

And it did:

Net cost per row over 2 million rows, lower is better:

	first C	new C	rusqlite	sqlite-loadable
Mixed data	67 ns	19 ns	46 ns	41 ns
Integers only	25 ns	16 ns	36 ns	32 ns
Decimals only	106 ns	18 ns	55 ns	47 ns

Finally, I had it rewrite the prototype as a research report to match the others in simonw/research by prompting this:

Clone simonw/research from GitHub and turn your above investigation into a research report as seen in that repository - you can give me a zip file to download that contains your new folder

Here's the resulting report.