"""phase21 — composite performance indexes

Adds composite (multi-column) indexes on the highest-traffic query patterns.
Phase 12 already covers single-column indexes; these target the multi-column
WHERE clauses that appear on every Reports, Inspections list, and Issues list
page load.

  inspections (facility_id, inspection_date)
    — facility-scoped date-range queries on every list page and report

  inspections (inspector_id, inspection_date)
    — inspector-scoped date-range queries on the Performance page and API stats

  inspections (status, inspection_date)
    — "completed inspections in date range" pattern used by all score aggregations

  issues (facility_id, status)
    — "open issues at this facility" pattern used by reports and dashboard

All existence checks use INFORMATION_SCHEMA.STATISTICS — safe to re-run on
any MySQL version (compatible back to 5.7).
"""

import sqlalchemy as sa
from alembic import op

revision      = 'phase21_performance_indexes'
down_revision = 'phase21_template_active'
branch_labels = None
depends_on    = None


def _index_exists(bind, table: str, index_name: str) -> bool:
    result = bind.execute(sa.text(
        "SELECT COUNT(*) FROM information_schema.statistics "
        "WHERE table_schema = DATABASE() "
        "  AND table_name   = :table "
        "  AND index_name   = :index"
    ), {'table': table, 'index': index_name})
    return result.scalar() > 0


# (table, index_name, columns)
INDEXES = [
    ('inspections', 'ix_inspections_facility_date',   'facility_id, inspection_date'),
    ('inspections', 'ix_inspections_inspector_date',  'inspector_id, inspection_date'),
    ('inspections', 'ix_inspections_status_date',     'status, inspection_date'),
    ('issues',      'ix_issues_facility_status',      'facility_id, status'),
]


def upgrade():
    bind = op.get_bind()
    for table, index_name, columns in INDEXES:
        if not _index_exists(bind, table, index_name):
            op.execute(sa.text(
                f'CREATE INDEX {index_name} ON {table} ({columns})'
            ))


def downgrade():
    bind = op.get_bind()
    for table, index_name, _columns in INDEXES:
        if _index_exists(bind, table, index_name):
            op.execute(sa.text(
                f'DROP INDEX {index_name} ON {table}'
            ))