"""Pin allocation recompute to a day-local TiDB index range.

Revision ID: 458
Revises: 457
Create Date: 2026-08-10

``cmdb.cost.nightly_recompute_allocation`` previously walked cost facts
with a global id cursor plus a 90-day date predicate.  Production TiDB
cancelled that query with error 8176 when it crossed a large block of
out-of-window history.  The service now scans one accounting day at a
time; this index makes each ``date = :day AND id > :cursor`` page a
narrow ordered range instead of a historical-table scan.

The guard keeps the migration safe for already-repaired environments and
the TiDB DDL is online.
"""
from __future__ import annotations

import sqlalchemy as sa

from alembic import op

revision = "458"
down_revision = "457"
branch_labels = None
depends_on = None

_TABLE = "cmdb_cost_facts"
_INDEX = "ix_facts_date_id"


def _index_exists(bind, table: str, index: str) -> bool:
    return bind.execute(
        sa.text(
            "SELECT 1 FROM information_schema.STATISTICS "
            "WHERE TABLE_SCHEMA = DATABASE() "
            "  AND TABLE_NAME = :t AND INDEX_NAME = :i LIMIT 1"
        ),
        {"t": table, "i": index},
    ).scalar() is not None


def upgrade() -> None:
    bind = op.get_bind()
    if not _index_exists(bind, _TABLE, _INDEX):
        op.create_index(_INDEX, _TABLE, ["date", "id"])


def downgrade() -> None:
    bind = op.get_bind()
    if _index_exists(bind, _TABLE, _INDEX):
        op.drop_index(_INDEX, table_name=_TABLE)