asyncpg: Slow introspection when using multiple custom types in a query

• asyncpg version: 0.20.1
• PostgreSQL version: 12.1
• Do you use a PostgreSQL SaaS? If so, which? Can you reproduce the issue with a local PostgreSQL install?: postgresql is installed locally on archlinux
• Python version: 3.7.6 and 3.8.1
• Platform: Linux 5.4.13-arch1-1
• Do you use pgbouncer?: no
• Did you install asyncpg with pip?: yes
• If you built asyncpg locally, which version of Cython did you use?: N/A
• Can the issue be reproduced under both asyncio and uvloop?: yes

I have a few custom types (CREATE TYPE ...) in my database, and I’m running into issues where asyncpg’s introspection stage on queries using 2 or more of these types are taking > 1 second to complete.

e.g.

add log_min_duration_statement = 500 to the default postgresql.conf

Create a database test with schema:

CREATE TYPE FOURBIGINTS AS (i0 BIGINT, i1 BIGINT, i2 BIGINT, i3 BIGINT);
CREATE TYPE NUMBERS_AS_WORDS AS ENUM (
    'zero', 'one', 'two', 'three', 'four'
);
CREATE TABLE bigthings (
    thing_id BIGSERIAL PRIMARY KEY,
    num FOURBIGINTS,
    words NUMBERS_AS_WORDS
);

and run the following code:

import asyncio
import asyncpg

async def main():
    con_args = {
        'dsn': 'postgres:///test',
        'ssl': None,
        'min_size': 10,
        'max_size': 10
    }
    pool = await asyncpg.create_pool(**con_args)

    async with pool.acquire() as con:
        await con.execute(
            """\
        INSERT INTO bigthings
        (num, words)
        VALUES ($1::FOURBIGINTS, $2::NUMBERS_AS_WORDS)
        """,
            (0, 0, 0, 0), 'one')

asyncio.run(main())

watching the logs (on my system sudo journalctl -u postgresql -f, will show something like:

2020-01-21 14:45:49.066 CET [118544] LOG:  duration: 1593.511 ms  execute __asyncpg_stmt_2__: WITH RECURSIVE typeinfo_tree(
            oid, ns, name, kind, basetype, has_bin_io, elemtype, elemdelim,
            range_subtype, elem_has_bin_io, attrtypoids, attrnames, depth)
        AS (
            SELECT
...
2020-01-21 14:45:49.066 CET [118544] DETAIL:  parameters: $1 = '{16584,16582}'

I’ve traced this back to the call to _introspect_types in connection.py.

From a bit of testing, it only happens if there are multiple custom types used in the query. e.g. if i change the query to simply be INSERT INTO bigthings (num) VALUES ($1::FOURBIGINTS), then everything is nice and fast as expected, or if i change the bigthings.words column to a VARCHAR, then there is no problem. But as soon as I include two or more custom types (e.g. 2 enum types or 2 tuple types, or a mix) then I see the slow downs.

Is there anything I can do to either remove the need for this introspection (e.g. giving asyncpg some hints about these types), or maybe I’m doing something wrong that I can correct?