That's not entirely true. C/C++ code can certainly be compiled down to WASM, but you cannot interface with the operating system as you would in a normal C/C++ program. To get around that restriction postgres-wasm ships an entire Linux distribution that is run inside the browser. This comes with an immense performance penalty.

To get an impression of the performance penalty, just run the following query:

  SELECT SUM(i) FROM generate_series(0, 1000000, 1) tbl(i);

This simple query completes in 100ms locally on my laptop, but takes 17265ms in postgres-wasm. That is a slowdown of 170x.

Now that is not WASM's fault - when running the same query in duckdb-wasm [1] on my laptop the query takes 10ms using WASM, and 5ms when run locally, with a slow-down of only a factor of 2. But in order to achieve those results we did have to adopt the DuckDB codebase to compile natively to WASM. That is absolutely possible but it does take engineering effort - particularly when it comes to larger older projects that are not designed from the ground up with this in mind.

[1] https://shell.duckdb.org