Make rel->query work with a read only connection

Author: evertlammerts

src/duckdb_py/pyrelation.cpp

@@ -1548,7 +1548,7 @@ static bool IsDescribeStatement(SQLStatement &statement) {
 }
 
 unique_ptr<DuckDBPyRelation> DuckDBPyRelation::Query(const string &view_name, const string &sql_query) {
-	auto view_relation = CreateView(view_name);
+	rel->CreateView(view_name, /*replace=*/true, /*temporary=*/true);
 	auto all_dependencies = rel->GetAllDependencies();
 
 	Parser parser(rel->context->GetContext()->GetParserOptions());

tests/fast/api/test_528_rel_query_readonly.py

@@ -0,0 +1,76 @@
+"""Regression test for duckdblabs/motherduck#528.
+
+`DuckDBPyRelation.query(view_name, sql)` internally calls CreateView, which
+writes to the default catalog. On a read-only attached database the write
+fails with InvalidInputException, breaking any user pattern that uses
+rel.query() against a read-only database (typical for MotherDuck
+read-scaling tokens or any duckdb.connect(path, read_only=True)).
+
+`rel.select()` and `conn.sql()` don't create a view and work fine — only
+rel.query() trips the bug.
+"""
+
+from __future__ import annotations
+
+import duckdb
+
+
+def test_rel_query_on_readonly_database(tmp_path):
+    db_path = tmp_path / "readonly.duckdb"
+
+    # Step 1: create the database with test data using a writable connection
+    with duckdb.connect(str(db_path)) as setup_conn:
+        setup_conn.execute(
+            """
+            CREATE TABLE orders AS
+            SELECT * FROM (
+                VALUES (1, 'A', 100), (2, 'B', 250), (3, 'C', 50)
+            ) AS t(order_id, product, quantity)
+            """
+        )
+
+    # Step 2: reopen read-only and exercise rel.query()
+    conn = duckdb.connect(str(db_path), read_only=True)
+    try:
+        rel = conn.sql("SELECT * FROM orders")
+        result = rel.query(
+            "duckdb_settings()",
+            "SELECT value FROM duckdb_settings() WHERE name = 'TimeZone'",
+        ).fetchone()
+        assert result is not None
+        assert isinstance(result[0], str)  # value column is a string
+    finally:
+        conn.close()
+
+
+def test_rel_select_on_readonly_database_still_works(tmp_path):
+    """Sanity: rel.select() (which doesn't create a view) must continue to work."""
+    db_path = tmp_path / "readonly.duckdb"
+    with duckdb.connect(str(db_path)) as setup_conn:
+        setup_conn.execute("CREATE TABLE t AS SELECT 1 AS x")
+
+    conn = duckdb.connect(str(db_path), read_only=True)
+    try:
+        rel = conn.sql("SELECT * FROM t")
+        result = rel.select(
+            duckdb.FunctionExpression("current_setting", duckdb.ConstantExpression("TimeZone"))
+        ).fetchone()
+        assert result is not None
+        assert isinstance(result[0], str)
+    finally:
+        conn.close()
+
+
+def test_conn_sql_on_readonly_database_still_works(tmp_path):
+    """Sanity: conn.sql() (no view created) must continue to work."""
+    db_path = tmp_path / "readonly.duckdb"
+    with duckdb.connect(str(db_path)) as setup_conn:
+        setup_conn.execute("CREATE TABLE t AS SELECT 1 AS x")
+
+    conn = duckdb.connect(str(db_path), read_only=True)
+    try:
+        result = conn.sql("SELECT value FROM duckdb_settings() WHERE name = 'TimeZone'").fetchone()
+        assert result is not None
+        assert isinstance(result[0], str)
+    finally:
+        conn.close()