fix(db_engine_specs): stop filtering out Postgres schemas prefixed with pg

SQLAlchemy's Postgres dialect excludes system schemas with
`nspname NOT LIKE 'pg_%'`, but `_` is a single-character wildcard in
SQL LIKE patterns, so it silently drops any user-defined schema that
starts with `pg` plus one more character (e.g. `pgsql`, `pgstats`),
not just the real `pg_` system schemas. Override `get_schema_names` in
PostgresBaseEngineSpec with a Python-side prefix check instead.

Fixes #30678

superset/db_engine_specs/postgres.py

@@ -793,6 +793,28 @@ WHERE datistemplate = false;
                 )
             }
 
+    @classmethod
+    def get_schema_names(cls, inspector: Inspector) -> set[str]:
+        """
+        Return all schema names, excluding actual Postgres system schemas.
+
+        SQLAlchemy's Postgres dialect filters out system schemas with the
+        query ``nspname NOT LIKE 'pg_%'``. Since ``_`` is a single-character
+        wildcard in SQL ``LIKE`` patterns, this unintentionally excludes any
+        user-defined schema that merely starts with ``pg`` followed by any
+        other character (e.g. ``pgsql``, ``pgstats``), not only the actual
+        Postgres system schemas, which are always prefixed with the literal
+        string ``pg_`` (e.g. ``pg_catalog``, ``pg_toast``).
+        """
+        with inspector.engine.connect() as conn:
+            return {
+                name
+                for (name,) in conn.execute(
+                    text("SELECT nspname FROM pg_namespace ORDER BY nspname")
+                )
+                if not name.startswith("pg_")
+            }
+
     @classmethod
     def get_table_names(
         cls, database: Database, inspector: PGInspector, schema: str | None

tests/unit_tests/db_engine_specs/test_postgres.py

@@ -403,3 +403,33 @@ def test_interval_type_mutator() -> None:
     assert mutator(True) is None
     assert mutator([1, 2, 3]) is None
     assert mutator({"days": 1}) is None
+
+
+def test_get_schema_names_excludes_only_actual_system_schemas(
+    mocker: MockerFixture,
+) -> None:
+    """
+    DB Eng Specs (postgres): Test ``get_schema_names``
+
+    User-defined schemas that merely start with ``pg`` (but are not
+    actual Postgres system schemas, which always start with the literal
+    ``pg_``) must not be filtered out. See SIP/issue #30678.
+    """
+    inspector = mocker.MagicMock()
+    inspector.engine.connect().__enter__().execute.return_value = [
+        ("public",),
+        ("pgsql",),
+        ("pgstats",),
+        ("pg_catalog",),
+        ("pg_toast",),
+        ("information_schema",),
+    ]
+
+    schemas = spec.get_schema_names(inspector)
+
+    assert schemas == {
+        "public",
+        "pgsql",
+        "pgstats",
+        "information_schema",
+    }