Question

对于产品表上的某些过滤子条件，我想找出这些过滤产品的不同类别（数百万种产品）。

明确的做法

use-> preferences.edit().remove("key").commit();

当涉及很多行时，

需要很长时间才能完成（相反，使用SELECT DISTINCT category_id FROM "products" WHERE _conditions_并没有太大变化）

根据https://wiki.postgresql.org/wiki/Loose_indexscan，当不同的col上只有相对少量的不同值时（就像这里有~30个类别的情况一样），条件

GROUP BY

可以重新表示为递归CTE

SELECT DISTINCT category_id FROM "products"

在我的用例中实际上表现更好（milisecons而不是秒）。但它没有条件。

如何添加＆＃39; WITH RECURSIVE t AS ( SELECT MIN(category_id) AS category_id FROM "products" UNION ALL SELECT (SELECT MIN(category_id) FROM "products" WHERE category_id > t.category_id) FROM t WHERE t.category_id IS NOT NULL ) SELECT category_id FROM t WHERE category_id IS NOT NULL UNION ALL SELECT NULL WHERE EXISTS(SELECT 1 FROM "products" WHERE category_id IS NULL);＆＃39;适当地适应递归CTE？

Answer 1

WITH RECURSIVE t AS (
   SELECT MIN(category_id) AS category_id FROM "products" WHERE _conditions_
   UNION ALL
   SELECT (SELECT MIN(category_id) FROM "products" WHERE category_id > t.category_id AND _conditions_ )
   FROM t WHERE t.category_id IS NOT NULL
   )
SELECT category_id FROM t WHERE category_id IS NOT NULL
UNION ALL
SELECT NULL WHERE EXISTS(SELECT 1 FROM "products" WHERE category_id IS NULL AND _conditions_);

喜欢这个。 条件中有三个。

松散的索引搜索postgres有条件

1 个答案: