"""Phase 12: Add performance indexes on high-filter columns

Revision ID: phase12_performance_indexes
Revises: phase11_director_role
Create Date: 2026-04-25

Rationale
---------
The following columns are filtered or ordered on every page load but had no
DB index, causing full table scans as row counts grow:

  inspections
    - status          — filtered on list/dashboard/SLA queries
    - facility_id     — filtered for customer-scoped views and reports
    - inspector_id    — filtered for inspector-scoped views
    - inspection_date — used in all trend/score queries (ORDER BY, range filter)

  issues
    - status          — filtered on every issues list/dashboard load
    - severity        — filtered in dashboard breakdown and issues list
    - assigned_to     — filtered for inspector-scoped views
    - reported_at     — used for ordering

Existence checks use information_schema so the migration is safe to re-run
on any MySQL version (compatible back to 5.7).
"""

from alembic import op
from sqlalchemy import text


def _index_exists(conn, table: str, index_name: str) -> bool:
    """Return True if the named index already exists on the given table."""
    result = conn.execute(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


revision      = 'phase12_performance_indexes'
down_revision = 'phase11_director_role'
branch_labels = None
depends_on    = None


# (table, index_name, column)
INDEXES = [
    ('inspections', 'ix_inspections_status',          'status'),
    ('inspections', 'ix_inspections_facility_id',     'facility_id'),
    ('inspections', 'ix_inspections_inspector_id',    'inspector_id'),
    ('inspections', 'ix_inspections_inspection_date', 'inspection_date'),
    ('issues',      'ix_issues_status',               'status'),
    ('issues',      'ix_issues_severity',             'severity'),
    ('issues',      'ix_issues_assigned_to',          'assigned_to'),
    ('issues',      'ix_issues_reported_at',          'reported_at'),
]


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


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