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

Abierto
#1,173 2 comentarios 0 reacciones 0 asignados Ver en GitHub

Nadie ha tomado este issue todavía.

Evaluación

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

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.

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.

Lenguaje dominante
Python
Estrellas
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Forks
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Merge medio
2 d 20 h
PR fusionados (30 d)
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