我在一些查询上遇到了 MySQL（5.5 版）左连接性能的问题。在所有情况下，我都能够通过使用联合和子选择重组查询来解决这个问题（我在《高性能 MySQL 》一书中看到了一些例子）。问题是这会导致非常混乱的查询。
下面是产生完全相同结果的两个查询的示例。第一个查询比第二个查询慢大约两个数量级。第二个查询比第一个查询可读性差得多。
据我所知，这些类型的查询并没有因为索引错误而表现不佳。在所有情况下，当我重组查询时，它运行得很好。我还尝试仔细查看索引并使用提示无济于事。
有没有其他人遇到过类似的 MySQL 问题？我应该尝试调整任何服务器参数吗？有没有人找到一种更清洁的方法来解决这类问题？
查询 1
select
i.id,
sum(vp.measurement * pol.quantity_ordered) measurement_on_order
from items i
left join (vendor_products vp, purchase_order_lines pol, purchase_orders po) on
vp.item_id = i.id and
pol.vendor_product_id = vp.id and
pol.purchase_order_id = po.id and
po.received_at is null and
po.closed_at is null
group by i.id
explain:
+----+-------------+-------+--------+-------------------------------+-------------------+---------+-------------------------------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+-------------------------------+-------------------+---------+-------------------------------------+------+-------------+
| 1 | SIMPLE | i | index | NULL | PRIMARY | 4 | NULL | 241 | Using index |
| 1 | SIMPLE | po | ref | PRIMARY,received_at,closed_at | received_at | 9 | const | 2 | |
| 1 | SIMPLE | pol | ref | purchase_order_id | purchase_order_id | 4 | nutkernel_dev.po.id | 7 | |
| 1 | SIMPLE | vp | eq_ref | PRIMARY,item_id | PRIMARY | 4 | nutkernel_dev.pol.vendor_product_id | 1 | |
+----+-------------+-------+--------+-------------------------------+-------------------+---------+-------------------------------------+------+-------------+
查询 2
select
i.id,
sum(on_order.measurement_on_order) measurement_on_order
from (
(
select
i.id item_id,
sum(vp.measurement * pol.quantity_ordered) measurement_on_order
from purchase_orders po
join purchase_order_lines pol on pol.purchase_order_id = po.id
join vendor_products vp on pol.vendor_product_id = vp.id
join items i on vp.item_id = i.id
where
po.received_at is null and po.closed_at is null
group by i.id
)
union all
(select id, 0 from items)
) on_order
join items i on on_order.item_id = i.id
group by i.id
explain:
+------+--------------+------------+--------+-------------------------------+--------------------------------+---------+-------------------------------------+------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+------+--------------+------------+--------+-------------------------------+--------------------------------+---------+-------------------------------------+------+----------------------------------------------+
| 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 3793 | Using temporary; Using filesort |
| 1 | PRIMARY | i | eq_ref | PRIMARY | PRIMARY | 4 | on_order.item_id | 1 | Using index |
| 2 | DERIVED | po | ALL | PRIMARY,received_at,closed_at | NULL | NULL | NULL | 20 | Using where; Using temporary; Using filesort |
| 2 | DERIVED | pol | ref | purchase_order_id | purchase_order_id | 4 | nutkernel_dev.po.id | 7 | |
| 2 | DERIVED | vp | eq_ref | PRIMARY,item_id | PRIMARY | 4 | nutkernel_dev.pol.vendor_product_id | 1 | |
| 2 | DERIVED | i | eq_ref | PRIMARY | PRIMARY | 4 | nutkernel_dev.vp.item_id | 1 | Using index |
| 3 | UNION | items | index | NULL | index_new_items_on_external_id | 257 | NULL | 3380 | Using index |
| NULL | UNION RESULT | <union2,3> | ALL | NULL | NULL | NULL | NULL | NULL | |
+------+--------------+------------+--------+-------------------------------+--------------------------------+---------+-------------------------------------+------+----------------------------------------------+