New way of deriving named parameters using explain, refs #1421

datasette/utils/__init__.py

@@ -1076,3 +1076,15 @@ def method(self, *args, **kwargs):
 
 class StartupError(Exception):
     pass
+
+
+_re_named_parameter = re.compile(":([a-zA-Z0-9_]+)")
+
+async def derive_named_parameters(db, sql):
+    explain = 'explain {}'.format(sql.strip().rstrip(";"))
+    possible_params = _re_named_parameter.findall(sql)
+    try:
+        results = await db.execute(explain, {p: None for p in possible_params})
+        return [row["p4"].lstrip(":") for row in results if row["opcode"] == "Variable"]
+    except sqlite3.DatabaseError:
+        return []

datasette/views/base.py

@@ -159,7 +159,6 @@ async def view(request, send):
 
 class DataView(BaseView):
     name = ""
-    re_named_parameter = re.compile(":([a-zA-Z0-9_]+)")
 
     async def options(self, request, *args, **kwargs):
         r = Response.text("ok")

datasette/views/database.py

@@ -10,6 +10,7 @@
 from datasette.utils import (
     await_me_maybe,
     check_visibility,
+    derive_named_parameters,
     to_css_class,
     validate_sql_select,
     is_url,
@@ -223,7 +224,9 @@ async def data(
             await self.check_permission(request, "execute-sql", database)
 
         # Extract any :named parameters
-        named_parameters = named_parameters or self.re_named_parameter.findall(sql)
+        named_parameters = named_parameters or await derive_named_parameters(
+            self.ds.get_database(database), sql
+        )
         named_parameter_values = {
             named_parameter: params.get(named_parameter) or ""
             for named_parameter in named_parameters

tests/test_utils.py

@@ -626,3 +626,18 @@ def test_parse_metadata(content, expected):
             utils.parse_metadata(content)
     else:
         assert utils.parse_metadata(content) == expected
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("sql,expected", (
+    ("select 1", []),
+    ("select 1 + :one", ["one"]),
+    ("select 1 + :one + :two", ["one", "two"]),
+    ("select 'bob' || '0:00' || :cat", ["cat"]),
+    ("select this is invalid", []),
+))
+async def test_derive_named_parameters(sql, expected):
+    ds = Datasette([], memory=True)
+    db = ds.get_database("_memory")
+    params = await utils.derive_named_parameters(db, sql)
+    assert params == expected