Call a Postgres function through the Supabase REST RPC endpoint (/rest/v1/rpc/{fn}), passing arguments correctly and handling PostgREST's structured error responses.

Documented steps

1. Every function in an exposed schema that the calling Postgres role can execute is available at POST https://<project_ref>.supabase.co/rest/v1/rpc/{function_name}. The /rpc/ prefix exists so function names don't collide with table/view routes.
2. Call with POST and a JSON object whose keys match the function's parameter names, e.g. for CREATE FUNCTION add_them(a integer, b integer) RETURNS integer: curl "https://<ref>.supabase.co/rest/v1/rpc/add_them" -X POST -H "apikey: <KEY>" -H "Content-Type: application/json" -d '{"a":1,"b":2}'.
3. If the function does NOT modify the database, it can also be called with GET, passing arguments as query parameters: curl "https://<ref>.supabase.co/rest/v1/rpc/add_them?a=1&b=2" -H "apikey: <KEY>". Parameters with a SQL DEFAULT can be omitted.
4. Functions that RETURNS SETOF <table_type> behave like a table/view route — chain the same filtering, ordering, limit/offset, and resource embedding query params, e.g. GET /rest/v1/rpc/get_planets?id=eq.1.
5. After creating or changing a function's signature or body, the PostgREST schema cache must refresh before /rpc/{fn} reflects it — on Supabase this happens automatically shortly after a change; if calls 404 immediately after DDL, that's the likely cause.
6. To pass an array argument via POST, use a JSON array value, e.g. for plus_one(arr int[]): -d '{"arr":[1,2,3,4]}'. Errors surface as JSON {"code":...,"message":...,"details":...,"hint":...} with an HTTP status derived from the Postgres error (nonexistent function → 404 PGRST202; raised exception without specific SQLSTATE → 400 P0001).
7. Control who can call the function independent of table grants: by default any role with EXECUTE can call it via RPC, so revoke execute from public/anon (or authenticated) if it should not be reachable via the Data API, and grant back only to roles that need it.
8. Docs: https://docs.postgrest.org/en/latest/references/api/functions.html https://docs.postgrest.org/en/latest/references/errors.html https://supabase.com/docs/guides/database/functions https://supabase.com/docs/guides/api/rest/postgrest-error-codes