Simon Sillesen
Full Stack Engineer at Factbird
Copenhagen, Capital Region of Denmark
Summary
🤩
Rockstar🎓
Top SchoolSimon Sillesen is a Full Stack Engineer based in Copenhagen with 12 years of hands-on experience building performant backend systems and shipping end-to-end products. He currently works at Factbird and has a track record at Netcompany, combining consulting discipline with product-focused engineering. An active open-source contributor in Rust, Simon has improved developer tooling and performance in high-profile projects like rust-analyzer, Apache Arrow (arrow-rs) and DataFusion, often adding benchmarks and low-level optimizations that avoid allocations and speed core operations. His background includes teaching computer architecture and an international master's exchange in Melbourne, reflecting a mix of practical production delivery and strong systems-level understanding. Colleagues can rely on him for pragmatic refactors that improve maintainability while squeezing out measurable performance gains.
11 years of coding experience
STX, Bioteknologi, STX, Bioteknologi at Sønderborg StatsskoleThe University of MelbourneBachelor's degree, Software engineering, Bachelor's degree, Software engineering at Aalborg Universitet
Danish, English, German
Github Skills (31)
algorithms10
benchmark10
benchmarking10
language-server-protocol10
data-structure10
fusion10
apache-arrow10
coreutils10
language-server10
performance-optimization10
data-structures10
command-line-tool10
query-engine10
rust10
code-formatting10
Programming languages (10)
Github contributions (5)
apache/datafusion
Jun 2023 - Mar 2025
Apache DataFusion SQL Query Engine
Role in this project:
Back-end Developer
Back-end DeveloperContributions:30 reviews, 18 PRs, 49 comments in 1 year 9 months
Contributions summary:Simon contributed to optimizing the Apache DataFusion query engine, focusing on performance improvements. They implemented metrics for the UnnestExec operator and added benchmarking for various functions, including date_bin, nullif, base64/hex decoding, iszero, signum, isnan, date_trunc, uuid, to_hex, and chr. The user also improved the efficiency of core functionalities, such as the `DFSchema::merge` method, and introduced a range table function.
querypythonquery-enginedataframerust
apache/arrow-rs
Apr 2023 - Nov 2024
Official Rust implementation of Apache Arrow
Role in this project:Back-end Developer
Back-end DeveloperContributions:6 reviews, 7 PRs, 17 comments in 1 year 6 months
Contributions summary:Simon primarily focused on optimizing and extending the Apache Arrow-rs codebase. Their contributions included optimizing boolean array aggregation functions, specifically `min_boolean`, `max_boolean`, and `bool_and`, leading to significant performance improvements. They also added benchmarks to assess performance and refactored the code to increase code maintainability. Additionally, the user supported Duration data types in the JSON reader.
rustolapapachearrowapache-arrow
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