fix: hasSingleUnnamedParam incorrectly matching named parameters

The hasSingleUnnamedParam function was only checking the parameter type
but not whether the parameter actually had no name. This caused functions
with a single NAMED parameter (e.g., `foo(data json)`) to incorrectly
match the single-param fallback mode.

The result was a confusing PostgreSQL error 42883 "function does not exist"
instead of a clean PGRST202 error explaining that no matching function
was found.

Added ppName == mempty check so functions with named parameters don't
incorrectly match the single-param fallback.

CHANGELOG.md

@@ -10,6 +10,8 @@ All notable changes to this project will be documented in this file. From versio
 
 ### Fixed
 
+- Fix `hasSingleUnnamedParam` incorrectly matching functions with named parameters by @joelonsql in #4553
+  + Functions with a single named parameter (e.g., `foo(data json)`) no longer incorrectly match the single-param fallback, returning a clean `PGRST202` error instead of a confusing PostgreSQL `42883` error.
 - Fix misleading logs on unsupported PostgreSQL versions by @taimoorzaeem in #4519
 - Fix regression where the `PGRST103` error response was truncated by @laurenceisla in #4455
   + Happened when an `offset` was greater than the rows requested and `Prefer: count=exact` was sent.

src/PostgREST/Plan.hs

@@ -256,14 +256,16 @@ findProc qi argumentsKeys allProcs contentMediaType isInvPost =
       | hasSingleUnnamedParam proc = (ts,proc:fs)
       | otherwise                  = (ts,fs)
     -- If the function is called with post and has a single unnamed parameter
-    -- it can be called depending on content type and the parameter type
-    hasSingleUnnamedParam Function{pdParams=[RoutineParam{ppType}]} = isInvPost && case (contentMediaType, ppType) of
-      (MTApplicationJSON, "json")  -> True
-      (MTApplicationJSON, "jsonb") -> True
-      (MTTextPlain, "text")        -> True
-      (MTTextXML, "xml")           -> True
-      (MTOctetStream, "bytea")     -> True
-      _                            -> False
+    -- it can be called depending on content type and the parameter type.
+    -- The parameter must have no declared name (ppName == mempty).
+    hasSingleUnnamedParam Function{pdParams=[RoutineParam{ppName, ppType}]} =
+      isInvPost && ppName == mempty && case (contentMediaType, ppType) of
+        (MTApplicationJSON, "json")  -> True
+        (MTApplicationJSON, "jsonb") -> True
+        (MTTextPlain, "text")        -> True
+        (MTTextXML, "xml")           -> True
+        (MTOctetStream, "bytea")     -> True
+        _                            -> False
     hasSingleUnnamedParam _ = False
     matchesParams proc =
       let

test/spec/Feature/Query/RpcSpec.hs

@@ -1146,12 +1146,24 @@ spec =
             }
 
       context "single unnamed param" $ do
-        it "can insert json directly" $
+        it "can insert json directly with unnamed parameter" $
           post "/rpc/unnamed_json_param"
               [json|{"A": 1, "B": 2, "C": 3}|]
             `shouldRespondWith`
               [json|{"A": 1, "B": 2, "C": 3}|]
 
+        it "rejects json body when single param has a name" $
+          post "/rpc/named_json_param"
+              [json|{"A": 1, "B": 2, "C": 3}|]
+            `shouldRespondWith`
+              [json|{
+                "code":"PGRST202",
+                "message":"Could not find the function test.named_json_param(A, B, C) in the schema cache",
+                "details":"Searched for the function test.named_json_param with parameters A, B, C or with a single unnamed json/jsonb parameter, but no matches were found in the schema cache.",
+                "hint":null
+              }|]
+              { matchStatus = 404 }
+
         it "can insert text directly" $ do
           request methodPost "/rpc/unnamed_text_param"
             [("Content-Type", "text/plain"), ("Accept", "text/plain")]

test/spec/fixtures/schema.sql

@@ -2356,6 +2356,11 @@ create or replace function test.unnamed_json_param(json) returns json as $$
   select $1;
 $$ language sql;
 
+-- Function with a NAMED json parameter (for testing single param fallback behavior)
+create or replace function test.named_json_param(data json) returns json as $$
+  select data;
+$$ language sql;
+
 create or replace function test.unnamed_text_param(text) returns "text/plain" as $$
   select $1::"text/plain";
 $$ language sql;