带有子查询的 SQL 查询执行得很糟糕

Question

我有一个很长的查询，它应该给我一些有关发货的信息，它有效，但它的性能非常糟糕。加载大约需要 4500 毫秒。

SELECT
    DATE(paid_at) AS day,
    COUNT(*) as order_count,
    (
      SELECT COUNT(*) FROM line_items
      WHERE order_id IN (SELECT id from orders WHERE DATE(paid_at) = day)
    ) as product_count,
    (
      SELECT COUNT(*) FROM orders
      WHERE shipping_method = 'colissimo'
      AND DATE(paid_at) = day
      AND state IN ('paid','shipped','completed')
    ) as orders_co,
    (
      SELECT COUNT(*) FROM orders
      WHERE shipping_method = 'colissimo'
      AND DATE(paid_at) = day
      AND state IN ('paid','shipped','completed')
      AND paid_amount < 70
    ) as co_less_70,
    (
      SELECT COUNT(*) FROM orders
      WHERE shipping_method = 'colissimo'
      AND DATE(paid_at) = day
      AND state IN ('paid','shipped','completed')
      AND paid_amount >= 70
    ) as co_plus_70,
    (
      SELECT COUNT(*) FROM orders
      WHERE shipping_method = 'mondial_relais'
      AND DATE(paid_at) = day
      AND state IN ('paid','shipped','completed')
    ) as orders_mr,
    (
      SELECT COUNT(*) FROM orders
      WHERE shipping_method = 'mondial_relais'
      AND DATE(paid_at) = day
      AND state IN ('paid','shipped','completed')
      AND paid_amount < 70
    ) as mr_less_70,
    (
      SELECT COUNT(*) FROM orders
      WHERE shipping_method = 'mondial_relais'
      AND DATE(paid_at) = day
      AND state IN ('paid','shipped','completed')
      AND paid_amount >= 70
    ) as mr_plus_70
    FROM orders
    WHERE MONTH(paid_at) = 11
    AND YEAR(paid_at) = 2011
    AND state IN ('paid','shipped','completed')
    GROUP BY day;

知道我可能做错了什么或者我可以做得更好吗？我还有其他类似长度的查询，但加载时间并不像这样。我认为这比每天进行单独的查询（在我的编程中而不是 SQL 查询中）要快。

Answer 1

这是因为您在不需要子查询的地方使用了子查询。

作为一般规则，如果主 SELECT 子句中有子查询，则该子查询将为主 SELECT 子句中的每一行查询其中的表一次 - 因此，如果您有 7 个子查询并选择日期范围在 30 天的时间里，您将有效地运行 210 个单独的子查询（加上您的主查询）。

（在某些情况下，某些查询优化器可以将子查询解析为主查询，但一般来说，您不能依赖于此。）

在这种情况下，您不需要任何 orders 子查询，因为您需要的所有 orders 数据都包含在主查询中 - 因此您可以将其重写为：

SELECT
    DATE(paid_at) AS day,
    COUNT(*) as order_count,
    (
      SELECT COUNT(*) FROM line_items
      WHERE order_id IN (SELECT id from orders WHERE DATE(paid_at) = day)
    ) as product_count,
    sum(case when shipping_method = 'colissimo' then 1 end) as orders_co,
    sum(case when shipping_method = 'colissimo' AND 
                  paid_amount < 70 then 1 end) as co_less_70,
    sum(case when shipping_method = 'colissimo' AND 
                  paid_amount >= 70 then 1 end) as co_plus_70,
    sum(case when shipping_method = 'mondial_relais' then 1 end) as orders_mr,
    sum(case when shipping_method = 'mondial_relais' AND 
                  paid_amount < 70 then 1 end) as mr_less_70,
    sum(case when shipping_method = 'mondial_relais' AND 
                  paid_amount >= 70 then 1 end) as mr_plus_70
    FROM orders
    WHERE MONTH(paid_at) = 11
    AND YEAR(paid_at) = 2011
    AND state IN ('paid','shipped','completed')
    GROUP BY day;

Answer 2

您的查询中的问题是一遍又一遍地扫描同一个表。 ORDER 表的所有扫描（在您的情况下选择）都可以转换为多个 SUM+CASE 或 COUNT+CASE，如 带有 count 和 case 语句的 SQL 查询。