如何使用许多JOIN提高查询性能

Question

我有一个查询(目的是创建一个视图),它使用一些连接来获取每一列.对于添加的每组连接,性能会快速降低(指数级？).

什么是使这个查询更快的好方法？请查看查询中的注释.

如果有帮助,这是使用WordPress数据库架构.

这是EXPLAIN的屏幕截图

产品表

+--+----+
|id|name|
+--+----+
|1 |test|
+--+----+

元数据表

+----------+--------+-----+
|product_id|meta_key|value|
+----------+--------+-----+
|1         |price   |9.99 |
+----------+--------+-----+
|1         |sku     |ABC  |
+----------+--------+-----+

TERM_RELATIONSHIPS表

+---------+----------------+
|object_id|term_taxonomy_id|
+---------+----------------+
|1        |1               |
+---------+----------------+
|1        |2               |
+---------+----------------+

TERM_TAXONOMY表

+----------------+-------+--------+
|term_taxonomy_id|term_id|taxonomy|
+----------------+-------+--------+
|1               |1      |size    |
+----------------+-------+--------+
|2               |2      |stock   |
+----------------+-------+--------+

条款表

+-------+-----+
|term_id|name |
+-------+-----+
|1      |500mg|
+-------+-----+
|2      |10   |
+-------+-----+

QUERY

SELECT 
  products.id,
  products.name,
  price.value AS price,
  sku.value AS sku,
  size.name AS size
FROM products

/* These joins are performing quickly */

INNER JOIN `metadata` AS price ON products.id = price.product_id AND price.meta_key = 'price'
INNER JOIN `metadata` AS sku ON products.id = sku.product_id AND sku.meta_key = 'sku'

/* Here's the part that is really slowing it down - I run this chunk about 5 times with different strings to match */

INNER JOIN `term_relationships` AS tr ON products.id = tr.object_id
  INNER JOIN `term_taxonomy` AS tt
  ON tr.term_taxonomy_id = tt.term_taxonomy_id AND tt.taxonomy = 'size'
    INNER JOIN `terms` AS size
    ON tt.term_id = size.term_id

Answer 1

您的性能问题很可能是由与'term_taxonomy'表的连接引起的.
所有其他连接似乎都使用主键(您可能在其上有工作索引).

所以我的建议是在term_taxonomy_id 和 term_id上添加复合索引 (或者如果你必须:分类法).像这样:

CREATE UNIQUE INDEX idx_term_taxonomy_id_taxonomy
ON term_taxonomy( term_taxonomy_id, taxonomy);

希望这会帮助你.