"""phase15 — add indexes on audit_logs action and entity_type

The audit log filter UI allows filtering by action and entity_type.
Without indexes, every filter invocation performs a full table scan.
As the log grows toward the 180/365-day purge threshold this degrades
noticeably.  This migration adds individual indexes on both columns.

A composite index on (action, entity_type, created_at) would be ideal
for the combined-filter case, but individual indexes are added here to
keep the migration additive and safe for re-run.  created_at already
has an index from the model definition.

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

Revision ID: phase15_audit_log_indexes
Revises: phase14_facility_created_at
"""

revision      = 'phase15_audit_log_indexes'
down_revision = 'phase14_facility_created_at'
branch_labels = None
depends_on    = None

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


# (table, index_name, column)
INDEXES = [
    ('audit_logs', 'ix_audit_logs_action',      'action'),
    ('audit_logs', 'ix_audit_logs_entity_type', 'entity_type'),
]


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}'
            ))