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"""Add indexes for CASCADE deletes on task_instance

Some databases don't add indexes on the FK columns so we have to add them for performance on CASCADE deletes.

Revision ID: f5fcbda3e651
Revises: 3c94c427fdf6
Create Date: 2022-06-15 18:04:54.081789

"""

from alembic import context, op

# revision identifiers, used by Alembic.
revision = 'f5fcbda3e651'
down_revision = '3c94c427fdf6'
branch_labels = None
depends_on = None
airflow_version = '2.3.3'


def _mysql_tables_where_indexes_already_present(conn):
    """
    If user downgraded and is upgrading again, we have to check for existing
    indexes on mysql because we can't (and don't) drop them as part of the
    downgrade.
    """
    to_check = [
        ('xcom', 'idx_xcom_task_instance'),
        ('task_reschedule', 'idx_task_reschedule_dag_run'),
        ('task_fail', 'idx_task_fail_task_instance'),
    ]
    tables = set()
    for tbl, idx in to_check:
        if conn.execute(f"show indexes from {tbl} where Key_name = '{idx}'").first():
            tables.add(tbl)
    return tables


def upgrade():
    """Apply Add indexes for CASCADE deletes"""
    conn = op.get_bind()
    tables_to_skip = set()

    # mysql requires indexes for FKs, so adding had the effect of renaming, and we cannot remove.
    if conn.dialect.name == 'mysql' and not context.is_offline_mode():
        tables_to_skip.update(_mysql_tables_where_indexes_already_present(conn))

    if 'task_fail' not in tables_to_skip:
        with op.batch_alter_table('task_fail', schema=None) as batch_op:
            batch_op.create_index('idx_task_fail_task_instance', ['dag_id', 'task_id', 'run_id', 'map_index'])

    if 'task_reschedule' not in tables_to_skip:
        with op.batch_alter_table('task_reschedule', schema=None) as batch_op:
            batch_op.create_index('idx_task_reschedule_dag_run', ['dag_id', 'run_id'])

    if 'xcom' not in tables_to_skip:
        with op.batch_alter_table('xcom', schema=None) as batch_op:
            batch_op.create_index('idx_xcom_task_instance', ['dag_id', 'task_id', 'run_id', 'map_index'])


def downgrade():
    """Unapply Add indexes for CASCADE deletes"""
    conn = op.get_bind()

    # mysql requires indexes for FKs, so adding had the effect of renaming, and we cannot remove.
    if conn.dialect.name == 'mysql':
        return

    with op.batch_alter_table('xcom', schema=None) as batch_op:
        batch_op.drop_index('idx_xcom_task_instance')

    with op.batch_alter_table('task_reschedule', schema=None) as batch_op:
        batch_op.drop_index('idx_task_reschedule_dag_run')

    with op.batch_alter_table('task_fail', schema=None) as batch_op:
        batch_op.drop_index('idx_task_fail_task_instance')
