"""Index the cloud daily-billing ingest watermark.

Adds ``ix_billing_daily_fetched_at`` on
``cmdb_cloud_billing_daily(fetched_at)``, which backs the External API's
incremental-pull signal (``GET /open/cloud/billing/changes`` and
``?updated_since=``): "which (subscription, day) cells were re-ingested
since T".

Why ``fetched_at`` and not ``updated_at`` — measured, not assumed
(2026-08-15, on the real upsert path):

    fetched_at  17:45:21 → 17:45:23  advanced
    updated_at  17:45:21 → 17:45:21  frozen

The daily-billing ingest writes through ``INSERT … ON DUPLICATE KEY
UPDATE`` with ``fetched_at`` in the SET list, while ``updated_at`` only
carries SQLAlchemy's client-side ``onupdate`` (never applied on that
path) and the DDL has no ``ON UPDATE CURRENT_TIMESTAMP``. Indexing
``updated_at`` would have produced an incremental feed that silently
returns nothing.

Note the sibling table is the mirror image: ``cmdb_cost_facts`` puts
``updated_at=func.now()`` in its SET list and leaves ``fetched_at``
alone, so the facts-side signal reads ``updated_at``. That query is
already bounded by ``(scope_id, date)`` (``ix_facts_scope_date``), so it
needs no new index — deliberately, since an index on a multi-million-row
table is not something to add for a read path that is already covered.

Revision ID: 471
Revises: 470
"""
from alembic import op

revision = "471"
down_revision = "470"
branch_labels = None
depends_on = None

INDEX_NAME = "ix_billing_daily_fetched_at"
TABLE = "cmdb_cloud_billing_daily"


def upgrade() -> None:
    op.create_index(INDEX_NAME, TABLE, ["fetched_at"])


def downgrade() -> None:
    op.drop_index(INDEX_NAME, table_name=TABLE)