如果我理解正确，您可以使用窗口函数和聚合：

select customer_id, product_ts, count(*) as num_products_before
from (select t.*,
             min(case when product = :product then timestamp end) over (partition by customerid) as product_ts
      from t
     ) t
where timestamp < product_ts
group by customer_id, product_ts;