MagicStack / MagicStack/asyncpg

No rows returned by fetch() when for DELETE rewritten to UPDATE using rule

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Lenguaje dominante
Python
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Descripción

  • asyncpg version: 0.29.0
  • PostgreSQL version: 16.2 (postgres:latest Docker image)
  • Python version: 3.12.3
  • Platform: Tested on macOS and Linux
  • Do you use pgbouncer?: No
  • Did you install asyncpg with pip?: Yes
  • Can the issue be reproduced under both asyncio and uvloop?: Yes

Unexpected behaviour: It seems asyncpg doesn't return the rows returned by a DELETE query rewritten to an UPDATE query by a rule. Perhaps because it's optimizing (the query status is DELETE 0, so perhaps it thinks it doesn't need to return any rows) or something like that? I didn't dive in any further to check if that is indeed what is happening.

Reproduction
import asyncio
import asyncpg

async def main():
    connection = await asyncpg.connect('postgresql://postgres:password@localhost/test')

    async def fetch_print(query, *params):
        result = await connection.fetch(query, *params)
        i = 0
        for row in result:
            print(f'  - {row}')
            i += 1
        if i == 0:
            print('  (no rows returned)')
        print('')

    # Create table with rule for deletion
    await connection.execute('''
        CREATE TABLE items (
            id      serial  PRIMARY KEY,
            name    text    UNIQUE,
            deleted boolean DEFAULT false
        );
        CREATE RULE softdelete AS
            ON DELETE
            TO items
            DO INSTEAD
                UPDATE items SET deleted = true WHERE id = OLD.id RETURNING OLD.*
        ;
        INSERT INTO items (name) VALUES
            ('foo'),
            ('bar')
        ;
    ''')

    print('Our table has a rule that updates the "deleted" column instead of deleting the row.\n')

    # Show contents
    print('We start with 2 rows which are not soft-deleted:')
    await fetch_print('SELECT * FROM items')

    # Try delete (the unexpected case)
    print('Deleting with RETURNING should give us a row (it does in psql), but we do not get any in asyncpg:')
    await fetch_print('''
        DELETE FROM items WHERE name = $1 RETURNING id
    ''', 'foo')

    # Confirm above query worked
    print('But the row is now soft-deleted:')
    await fetch_print('SELECT * FROM items')

    # Workaround
    print('If wrapped in a CTE it does work:')
    await fetch_print('''
        WITH x AS (
            DELETE FROM items WHERE name = $1 RETURNING id
        ) SELECT * FROM x
    ''', 'bar')

    # Confirm above query worked
    print('And it is again properly softdeleted:')
    await fetch_print('SELECT * FROM items')

    print('We now delete the rule.\n')
    await connection.execute('DROP RULE softdelete ON items')

    # Confirm normal delete without rule returns rows
    print('Normal deletion (without the rule) does return rows correctly:')
    await fetch_print('''
        DELETE FROM items RETURNING id
    ''')

    # Confirm above query worked
    print('And now both rows are indeed gone:')
    await fetch_print('SELECT * FROM items')

    # Clean up table after we are done
    await connection.execute('DROP TABLE items')

    await connection.close()

asyncio.run(main())
Output of reproduction
Our table has a rule that updates the "deleted" column instead of deleting the row.

We start with 2 rows which are not soft-deleted:
  - <Record id=1 name='foo' deleted=False>
  - <Record id=2 name='bar' deleted=False>

Deleting with RETURNING should give us a row (it does in psql), but we do not get any in asyncpg:
  (no rows returned)

But the row is now soft-deleted:
  - <Record id=2 name='bar' deleted=False>
  - <Record id=1 name='foo' deleted=True>

If wrapped in a CTE it does work:
  - <Record id=2>

And it is again properly softdeleted:
  - <Record id=1 name='foo' deleted=True>
  - <Record id=2 name='bar' deleted=True>

We now delete the rule.

Normal deletion (without the rule) does return rows correctly:
  - <Record id=1>
  - <Record id=2>

And now both rows are indeed gone:
  (no rows returned)

In contrast the psql command line tool does show me the resulting rows when the result code is DELETED 0.

Output of psql
test=# DELETE FROM items WHERE name = 'foo' RETURNING id;
 id 
----
  1
(1 row)

DELETE 0

So it is a bit unexpected that asyncpg doesn't return any rows.

Guía de contribución

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Primeros pasos

  1. Lee el issue completo y luego la guía de contribución del proyecto.
  2. Comenta en el issue que vas a ocuparte — evita que dos personas hagan lo mismo.
  3. Haz un fork del repositorio y trabaja en una rama.
  4. Abre un pull request que haga referencia al número del issue.

Línea de trabajo

Empieza ejecutando la reproducción en Python del issue contra PostgreSQL 16.2 y comparando el DELETE con RETURNING reescrito por una regla, la solución alternativa con CTE y un DELETE normal. Rastrea el resultado de fetch de asyncpg para la consulta reescrita directamente; se considera terminado cuando devuelve la fila mostrada por psql y conserva el comportamiento existente para los demás casos.

Escrito por el modelo de indexación a partir del texto del issue.

Evaluación

Stack tecnológico
postgresql, python
Área
databases
Tipo de issue
Error
Dificultad
3/5
Tiempo estimado
1-2 días
Estado de actividad
Estancado
Claridad
Bastante claro
Aptitud para principiantes
38/100

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