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migrations/versions/phase12_performance_indexes.py

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+"""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}'
+            ))