fix: Performance and high memory usage of relation hint calculation

* Calculation of hint message when requested relation is not present in schema cache requires creation of a FuzzySet (to use fuzzy search to find candidate tables). For schemas with many tables it is costly.
This patch introduces dbTablesFuzzyIndex in SchemaCache to memoize the FuzzySet creation.

* Additionally, because of FuzzySet large memory requirements, this patch introduces a limit of 500 relations per schema, above which FuzzySet is not created and hint calculation disabled.

test/io/fixtures/big_schema.sql

@@ -11375,12 +11375,34 @@ ALTER TABLE ONLY apflora.zielber
 
 ALTER TABLE apflora."user" ENABLE ROW LEVEL SECURITY;
 
+
+CREATE SCHEMA fuzzysearch;
+
+-- Create many tables to test fuzzy string search
+-- computing hints for non existing tables
+DO
+$$
+DECLARE
+  r record;
+BEGIN
+  FOR r IN
+    SELECT
+      format('CREATE TABLE fuzzysearch.unknown_table_%s ()', n) AS ct
+    FROM
+      generate_series(1, 499) n
+    LOOP
+      EXECUTE r.ct;
+    END LOOP;
+END
+$$;
+
 DROP ROLE IF EXISTS postgrest_test_anonymous;
 CREATE ROLE postgrest_test_anonymous;
 
 GRANT postgrest_test_anonymous TO :PGUSER;
 
 GRANT USAGE ON SCHEMA apflora TO postgrest_test_anonymous;
+GRANT USAGE ON SCHEMA fuzzysearch TO postgrest_test_anonymous;
 
 GRANT ALL PRIVILEGES ON ALL TABLES IN SCHEMA apflora
 TO postgrest_test_anonymous;

test/io/test_big_schema.py

@@ -70,3 +70,23 @@ def test_should_not_fail_with_stack_overflow(defaultenv):
         assert response.status_code == 404
         data = response.json()
         assert data["code"] == "PGRST205"
+
+
+def test_second_request_for_non_existent_table_should_be_quick(defaultenv):
+    "requesting a non-existent relationship should be quick after the fuzzy search index is loaded (2nd request)"
+
+    env = {
+        **defaultenv,
+        "PGRST_DB_SCHEMAS": "fuzzysearch",
+        "PGRST_DB_POOL": "2",
+        "PGRST_DB_ANON_ROLE": "postgrest_test_anonymous",
+    }
+
+    with run(env=env, wait_max_seconds=30) as postgrest:
+        response = postgrest.session.get("/unknown-table")
+        assert response.status_code == 404
+        data = response.json()
+        assert data["code"] == "PGRST205"
+        first_duration = response.elapsed.total_seconds()
+        response = postgrest.session.get("/unknown-table")
+        assert response.elapsed.total_seconds() < first_duration / 10