根据字段值限制查询结果

Question

我有一个具有休闲结构的表account ：

| agg_type  | agg_id  | sequence | payload | is_snapshot | timestamp |
| "account" | "agg_1" | 1        | "..."   | false       | ...       |
| "account" | "agg_1" | 2        | "..."   | true        | ...       |
| "account" | "agg_1" | 3        | "..."   | false       | ...       |
| "account" | "agg_1" | 4        | "..."   | false       | ...       |
| "account" | "agg_1" | 5        | "..."   | false       | ...       |
| "account" | "agg_1" | 6        | "..."   | false       | ...       |
| "account" | "agg_1" | 7        | "..."   | true        | ...       |
| "account" | "agg_1" | 8        | "..."   | false       | ...       |

我需要编写一个查询，该查询将从特定聚合的最新快照开始从该表中检索所有行。 例如，在此表的情况下，查询将返回最后两行（序列 7 和 8）。

我认为查询将 go 类似于

SELECT * FROM account 
WHERE
  agg_type='account'
  AND agg_id='agg_1'
ORDER BY sequence ASC
LIMIT (???);

这是我不太确定如何实施的(???)部分。

观察：

• 如果有任何帮助，我正在使用 Postgres。
• (agg_type, agg_id, sequence) 组合是主键。

Answer 1

简单地说，我们可以只检索序列大于或等于快照的最高序列 id 的所有帐户

SELECT * FROM account a
WHERE
  a.agg_type='account'
  AND a.agg_id='agg_1' 
  AND a.sequence >= 
    (SELECT MAX(sequence) FROM account b WHERE a.agg_type = b.agg_type AND a.agg_id = b. agg_id AND b.is_snapshot = true)

如果您想全部完成，则将其编写为联接可能会更清楚：

SELECT a.* 
FROM 
  account a
  INNER JOIN
  (
    SELECT 
      agg_type, 
      agg_id, 
      MAX(sequence) as maxseq 
    FROM account b 
    GROUP BY agg_type, add_id
  ) maxes
  ON 
    a.agg_type = maxes.agg_type and
    maxes.agg_id = a.max_id and
    a.sequence >= maxes.maxseq

这并不是说我们不能用任何一种形式完成任何一项任务（并且内部 postgres 可能无论如何都会执行它们），但我一直认为使用连接作为“这里有 10000 行，我想要只有符合这 1000 行规定的标准的 2000 行”最清楚地被认为是连接在一起的数据块

Answer 2

WITH a AS ( SELECT *,row_number() over(partition BY a.agg_type,a.agg_id ORDER BY a."SEQUENCE" DESC) rnk FROM account a ) SELECT * FROM a WHERE a.rnk <= 2;

Answer 3

一个 window function 可以为所有(agg_type, agg_id)组合仅使用一种排序：

with mark as (
  select *, 
         bool_or(is_snapshot) over w as trail_true
    from account
  window w as (partition by agg_type, agg_id 
                   order by sequence
            rows between 1 following
                     and unbounded following)
)
select *
  from mark
 where not coalesce(trail_true, false)
 order by agg_type, agg_id, sequence