Allow changing number of threads with env variable

Author: saulshanabrook

docs/reference/usage.md

@@ -22,6 +22,14 @@ pip install anywidget
 
 It follows [SPEC 0](https://scientific-python.org/specs/spec-0000/) in terms of what Python versions are supported.
 
+## Parallelism and threads
+
+The underlying Rust library uses Rayon for parallelism. You can control the worker thread count via the environment variable `RAYON_NUM_THREADS`. If this variable is not set or is invalid, the Python bindings default to using a single thread (`1`).
+
+```shell
+export RAYON_NUM_THREADS=4  # use 4 threads
+```
+
 (community)=
 
 ## Community

src/lib.rs

@@ -11,8 +11,13 @@ use pyo3::prelude::*;
 /// Bindings for egglog rust library
 #[pymodule]
 fn bindings(m: &Bound<'_, PyModule>) -> PyResult<()> {
+    // Configure Rayon thread pool from env var, defaulting to 1 if unset/invalid.
+    let num_threads = std::env::var("RAYON_NUM_THREADS")
+        .ok()
+        .and_then(|s| s.parse::<usize>().ok())
+        .unwrap_or(1);
     rayon::ThreadPoolBuilder::new()
-        .num_threads(1)
+        .num_threads(num_threads)
         .build_global()
         .unwrap();