# Buggy code
async def _get_statement(self, query, timeout, *, named: bool = False):
    statement = self._stmt_cache.get(query)
    if statement is not None:
        return statement

    # Only use the cache when:
    #  * `statement_cache_size` is greater than 0;
    #  * query size is less than `max_cacheable_statement_size`.
    use_cache = self._stmt_cache.get_max_size() > 0
    if (
        use_cache
        and self._config.max_cacheable_statement_size
        and len(query) > self._config.max_cacheable_statement_size
    ):
        use_cache = False

    if use_cache or named:
        stmt_name = self._get_unique_id("stmt")
    else:
        stmt_name = ""

    statement = await self._protocol.prepare(stmt_name, query, timeout)

    ready = statement._init_types()
    if ready is not True:
        if self._types_stmt is None:
            self._types_stmt = await self.prepare(self._intro_query)

        types = await self._types_stmt.fetch(list(ready))
        self._protocol.get_settings().register_data_types(types)

    if use_cache:
        self._stmt_cache.put(query, statement)

    # If we've just created a new statement object, check if there
    # are any statements for GC.
    if self._stmts_to_close:
        await self._cleanup_stmts()

    return statement


# Traceback (InvalidSQLStatementNameError)
In [1]: import os, asyncpg, asyncio

In [2]: loop = asyncio.get_event_loop()

In [3]: conn = loop.run_until_complete(asyncpg.connect(
...:     host = 'pgaas.mail.yandex.net',
...:     port = 12000,
...:     user = 'statinfra_api',
...:     database = 'statinfra_api_beta',
...:     password=os.environ['DB_PASSWORD'],
...:     ssl=True,
...:     statement_cache_size=0
...: ))

In [4]: loop.run_until_complete(conn.fetch('SELECT * FROM tasks'))
---------------------------------------------------------------------------
InvalidSQLStatementNameError              Traceback (most recent call last)
<ipython-input-5-8a44237acb87> in <module>()
----> 1 loop.run_until_complete(conn.fetch('SELECT * FROM tasks'))

/usr/lib/python3.6/asyncio/base_events.py in run_until_complete(self, future)
465             raise RuntimeError('Event loop stopped before Future completed.')
466
--> 467         return future.result()
468
469     def stop(self):

~/.virtualenvs/statbox-abt-backend/lib/python3.6/site-packages/asyncpg/connection.py in fetch(self, query, timeout, *args)
339         """
340         self._check_open()
--> 341         return await self._execute(query, args, 0, timeout)
342
343     async def fetchval(self, query, *args, column=0, timeout=None):

~/.virtualenvs/statbox-abt-backend/lib/python3.6/site-packages/asyncpg/connection.py in _execute(self, query, args, limit, timeout, return_status)
1186         timeout = self._protocol._get_timeout(timeout)
1187         with self._stmt_exclusive_section:
-> 1188             return await self._do_execute(query, executor, timeout)
1189
1190     async def _executemany(self, query, args, timeout):

~/.virtualenvs/statbox-abt-backend/lib/python3.6/site-packages/asyncpg/connection.py in _do_execute(self, query, executor, timeout, retry)
1197     async def _do_execute(self, query, executor, timeout, retry=True):
1198         if timeout is None:
-> 1199             stmt = await self._get_statement(query, None)
1200         else:
1201             before = time.monotonic()

~/.virtualenvs/statbox-abt-backend/lib/python3.6/site-packages/asyncpg/connection.py in _get_statement(self, query, timeout, named)
290                 self._types_stmt = await self.prepare(self._intro_query)
291
--> 292             types = await self._types_stmt.fetch(list(ready))
293             self._protocol.get_settings().register_data_types(types)
294

~/.virtualenvs/statbox-abt-backend/lib/python3.6/site-packages/asyncpg/prepared_stmt.py in fetch(self, timeout, *args)
155         :return: A list of :class:`Record` instances.
156         """
--> 157         data = await self.__bind_execute(args, 0, timeout)
158         return data
159

~/.virtualenvs/statbox-abt-backend/lib/python3.6/site-packages/asyncpg/prepared_stmt.py in __bind_execute(self, args, limit, timeout)
194         protocol = self._connection._protocol
195         data, status, _ = await protocol.bind_execute(
--> 196             self._state, args, '', limit, True, timeout)
197         self._last_status = status
198         return data

~/.virtualenvs/statbox-abt-backend/lib/python3.6/site-packages/asyncpg/protocol/protocol.pyx in bind_execute (asyncpg/protocol/protocol.c:66799)()

InvalidSQLStatementNameError: prepared statement "__asyncpg_stmt_1__" does not exist

# Fixed code
async def _get_statement(self, query, timeout, *, named: bool = False):
    statement = self._stmt_cache.get(query)
    if statement is not None:
        return statement

    # Only use the cache when:
    #  * `statement_cache_size` is greater than 0;
    #  * query size is less than `max_cacheable_statement_size`.
    use_cache = self._stmt_cache.get_max_size() > 0
    if (
        use_cache
        and self._config.max_cacheable_statement_size
        and len(query) > self._config.max_cacheable_statement_size
    ):
        use_cache = False

    if use_cache or named:
        stmt_name = self._get_unique_id("stmt")
    else:
        stmt_name = ""

    statement = await self._protocol.prepare(stmt_name, query, timeout)
    ready = statement._init_types()
    if ready is not True:
        types, intro_stmt = await self.__execute(
            self._intro_query, (list(ready),), 0, timeout
        )
        self._protocol.get_settings().register_data_types(types)
        if not intro_stmt.name and not statement.name:
            # The introspection query has used an anonymous statement,
            # which has blown away the anonymous statement we've prepared
            # for the query, so we need to re-prepare it.
            statement = await self._protocol.prepare(stmt_name, query, timeout)

    if use_cache:
        self._stmt_cache.put(query, statement)

    # If we've just created a new statement object, check if there
    # are any statements for GC.
    if self._stmts_to_close:
        await self._cleanup_stmts()

    return statement