Mock CTO interview: approaching a report query that takes 12 seconds

# ======================================================================
# Q: CTO: "A report query takes 12 seconds. Show me how you approach it."
#
# You: "I get the SQL with queryset.explain(analyze=True, buffers=True) or from
# pg_stat_statements. I look for a seq scan on a large table, a big gap between
# estimated and actual rows, and a sort that spills to disk. Usually the fix is
# a composite index with the equality column first, partial if we always filter
# by status. I build the index CONCURRENTLY. If the report doesn't need fresh
# data, I send it to the read replica or to a materialized view refreshed by
# Celery."
# ======================================================================

qs = (Order.objects.filter(company_id=cid, status='paid',
                           created_at__gte=since)
      .values('created_at__date').annotate(total=Sum('total')))
print(qs.explain(analyze=True, buffers=True))

class Meta:
    indexes = [models.Index(
        fields=['company', 'created_at'], include=['total'],
        condition=Q(status='paid'), name='order_paid_company_created')]