选择前 N 行，其中 TEXT 字段的长度总和达到某个限制

Question

我有一个 table 这样的：

CREATE TABLE cache (
  id BIGSERIAL PRIMARY KEY,
  source char(2) NOT NULL,
  target char(2) NOT NULL,
  q TEXT NOT NULL,
  result TEXT,
  profile TEXT NOT NULL DEFAULT '',
  created TIMESTAMP NOT NULL DEFAULT now(),
  api_engine text NOT NULL,
  encoded TEXT NOT NULL
);

我想传递 encoded 字段的列表（也许 OVER ... WINDOW ？）像这样的东西：

SELECT id, string_agg(encoded, '&q=') FROM cache

所以我会得到相应 ID 的列表，以及一串串接的字段 encoded: '&q=encoded1&q=encoded2&q=encoded3' ... 总长度不超过某个限制（比如不超过 2000 个字符）。

第二个条件，我想转到下一个window，当其中一个字段：源、目标或配置文件发生更改时。

如果可以在 FOR 循环中使用 SQL SELECT？

我知道如何用 plpgsql/plpython/plperl 做到这一点，但我想优化这个请求。

FOR rec IN
  SELECT array_agg(id) AS ids, string_agg(encoded, '&q=') AS url FROM cache
  WHERE result IS NULL
  ORDER BY source, target
LOOP
  -- here I call curl with that *url*

示例数据：

INSERT INTO cache (id, source, target, q, result, profile, api_engine, encoded) VALUES
   (1, 'ru', 'en', 'Длинная фраза по-русски'            , NULL, '', 'google', '%D0%94%D0%BB%D0%B8%D0%BD%D0%BD%D0%B0%D1%8F+%D1%84%D1%80%D0%B0%D0%B7%D0%B0+%D0%BF%D0%BE-%D1%80%D1%83%D1%81%D1%81%D0%BA%D0%B8')
 , (2, 'ru', 'es', 'Ещё одна непонятная фраза по-русски', NULL, '', 'google', '%D0%95%D1%89%D1%91+%D0%BE%D0%B4%D0%BD%D0%B0+%D0%BD%D0%B5%D0%BF%D0%BE%D0%BD%D1%8F%D1%82%D0%BD%D0%B0%D1%8F+%D1%84%D1%80%D0%B0%D0%B7%D0%B0+%D0%BF%D0%BE-%D1%80%D1%83%D1%81%D1%81%D0%BA%D0%B8')
-- etc...

等等，100500 行这样的。字段 source 和 target 可以是不同的语言代码，它们重复，所以我可能需要做 GROUP BY source, target, profile.

我想要 SELECT 前 N 行，其中字段编码与一些分隔符（如

的连接

&q=%D0%94%D0%BB%D0%B8%D0%BD%D0%BD%D0%B0%D1%8F+%D1%84%D1%80%D0%B0%D0%B7%D0%B0+%D0%BF%D0%BE-%D1%80%D1%83%D1%81%D1%81%D0%BA%D0%B8&q=%D0%95%D1%89%D1%91+%D0%BE%D0%B4%D0%BD%D0%B0+%D0%BD%D0%B5%D0%BF%D0%BE%D0%BD%D1%8F%D1%82%D0%BD%D0%B0%D1%8F+%D1%84%D1%80%D0%B0%D0%B7%D0%B0+%D0%BF%D0%BE-%D1%80%D1%83%D1%81%D1%81%D0%BA%D0%B8

所以这个连接字符串的长度不超过 (2000) 个字符。所以我将把那个字符串，以及那些行的所有 id，包含在 url 中（以相同的顺序，当然）。

然后我想 select 具有相同条件的下 N 行，依此类推。

Answer 1

您可以使用智能递归 CTE 来完成：

WITH RECURSIVE c AS ( -- 1st CTE is not recursive
   SELECT dense_rank()  OVER (ORDER BY     source, target, profile)             AS rnk
        , row_number()  OVER (PARTITION BY source, target, profile ORDER BY id) AS rn
        , lead(encoded) OVER (PARTITION BY source, target, profile ORDER BY id) AS next_enc
        , id, encoded
   FROM   cache
   )

 , rcte AS (  -- "recursion" starts here
   SELECT rnk, rn, ARRAY[id] AS ids, encoded AS url
        , CASE WHEN length(concat_ws('&q=', encoded || next_enc)) > 2000  -- max len
                 OR next_enc IS NULL  -- last in partition
               THEN TRUE END AS print
   FROM   c
   WHERE  rn = 1

   UNION ALL
   SELECT c.rnk, c.rn
        , CASE WHEN r.print THEN ARRAY[id] ELSE r.ids || c.id                      END AS ids
        , CASE WHEN r.print THEN c.encoded ELSE concat_ws('&q=', r.url, c.encoded) END AS url
        , CASE WHEN length(
             CASE WHEN r.print THEN concat_ws('&q=', c.encoded, c.next_enc)
                  ELSE concat_ws('&q=', r.url, c.encoded, c.next_enc) END) > 2000  -- max len
                 OR c.next_enc IS NULL  -- last in partition
               THEN TRUE END AS print
   FROM   rcte r
   JOIN        c USING (rnk)
   WHERE  c.rn = r.rn + 1
   )
SELECT ids, url
FROM   rcte
WHERE  print
ORDER  BY rnk, rn;

关于包含非递归 CTE 的 rCTE：

Multiple CTE in single query

但这可能是在 plpgsql 函数中循环实际上更快的罕见情况之一。

请参阅此相关答案以获取更多解释：

Grouping or Window

选择前 N 行，其中 TEXT 字段的长度总和达到某个限制

Selecting first N rows, where sum of lengths of TEXT field is up to some limit

sql

postgresql

select

aggregate-functions

window-functions