Question

问题：我正在Postgres上构建一个包含以下参数的RESTful API：

表格中特定记录的标识符<id>
过滤和排序参数（可选）
count或页面尺寸（可选）

例如：/presidents/16?filter=!isAlive&sort=lastName,givenNames&count=5

API返回count（或可能更少）记录，其中包含<id>指定的记录以及返回的offset和count条记录。

在上面的示例中，结果可能如下所示：

{
  "count": 5,
  "offset": 20,
  "records": [
    { "id": 17, "givenNames": "Andrew",    "lastName": "Johnson", "isAlive": false },
    { "id": 36, "givenNames": "Lyndon B.", "lastName": "Johnson", "isAlive": false },
    { "id": 35, "givenNames": "John F.",   "lastName": "Kennedy", "isAlive": false },
    { "id": 16, "givenNames": "Abraham",   "lastName": "Lincoln", "isAlive": false },
    { "id": 4,  "givenNames": "James",     "lastName": "Madison", "isAlive": false }
  ]
}

当前解决方案：我当前的方法是串行进行三次调用（将它们组合成一个嵌套查询，但效率问题仍然存在），我正在寻找更好的方法。

获取目标<id>引用的记录，在查询＃2中使用。
- e.g。 select * from presidents where id = 16
获取目标<id>
的偏移量
- e.g。 select count(*) from presidents where lastName < 'Lincoln' and givenNames < 'Abraham' and not isAlive order by lastName, givenNames, id
使用传递（或默认）offset和＃2中的limit计算相应的count和count(*)后，检索记录页
- e.g。 select * from presidents where not isAlive order by lastName, givenNames, id offset 20 limit 5

更新了SQL

我在下面的答案中提到了@ErwinBrandstetter所提供的内容，这与我在寻找怪物陈述方面的内容非常接近，并将其改为：

WITH prez AS (SELECT lastName, givenNames, id, isAlive FROM presidents WHERE not isAlive),
cte AS (SELECT * FROM prez WHERE id = 16),
start AS (SELECT (COUNT(*)/5)*5 as "offset" FROM prez WHERE (lastName, givenNames, id, isAlive) < (TABLE cte))
SELECT row_to_json(sub2) AS js
FROM  (
 SELECT (SELECT * FROM start) AS "offset"
       , count(*) AS "count"
       , array_agg(sub1) AS records
 FROM  (
    SELECT * from prez
    ORDER  BY lastName, givenNames, id
    OFFSET (SELECT * from start) LIMIT  5
    ) sub1
) sub2;

SQL Fiddle

Postgres是否有更简单的方法来确定给定查询的给定记录的偏移量？

相关问题：

Answer 1

您正在寻找的一站式商店：

WITH cte AS (SELECT lastName, givenNames, id AS x FROM presidents WHERE id = 16)
SELECT row_to_json(sub2) AS js
FROM  (
   SELECT (SELECT count(*) FROM presidents
           WHERE (lastName, givenNames, id) < (TABLE cte)) AS "offset"
         , count(*) AS "count"
         , array_agg(sub1) AS records
   FROM  (
      SELECT id, givenNames, lastName, isAlive
      FROM   presidents
      WHERE (lastName, givenNames, id) >= (TABLE cte)
      ORDER  BY lastName, givenNames, id
      LIMIT  5
      ) sub1
  ) sub2;

SQL Fiddle.

原始查询＃2 不正确：

SELECT * FROM presidents
WHERE lastName < 'Lincoln'
AND   givenNames < 'Abraham'
AND   id < 16 ...

要保留您的排序顺序，必须：

SELECT * FROM presidents
WHERE (lastName, givenNames, id) < ('Lincoln', 'Abraham', 16) ...

比较行类型，而不是在各列上使用AND表达式，这会产生完全不同的结果。这就是ORDER BY有效运作的方式。

除了(lastName, givenNames, id)上的PRIMARY KEY之外，您应该id row_number()以使快速。

变量与CTE中的`WITH prez AS ( SELECT lastName, givenNames, id, isAlive , row_number() OVER (ORDER BY lastName, givenNames, id) AS rn FROM presidents WHERE NOT isAlive ) , x AS (SELECT ((rn-1)/5)5 AS "offset" FROM prez WHERE id = 16) SELECT row_to_json(sub2) AS js FROM ( SELECT (SELECT "offset" FROM x) , count() AS "count" , array_agg(sub1) AS records FROM ( SELECT lastName, givenNames, id, isAlive FROM prez WHERE rn > (SELECT "offset" FROM x) ORDER BY rn LIMIT 5 ) sub1 ) sub2;`

根据您的更新要求。

EXPLAIN ANALYZE

multicolumn index

使用{{1}}测试效果。

在Postgres查询中确定目标记录的偏移量

1 个答案: