在 Ecto Elixir 中分页 union_all 查询

Question

我有 3 个表，我正在尝试对其进行 UNION_ALL 查询。这是查询。

compares =
  Compare
  |> join(:left, [comp], u in assoc(comp, :user))
  |> join(:left, [comp], c in assoc(comp, :camera))
  |> select([comp, u, c], %{
    title: comp.name,
    exid: comp.exid,
    table: "compares",
    requested_by: u.email,
    status: comp.status,
    camera_id: c.exid,
    created_at: comp.inserted_at
  })

timelapses =
  Timelapse
  |> join(:left, [tl], u in assoc(tl, :user))
  |> join(:left, [tl], c in assoc(tl, :camera))
  |> select([tl, u, c], %{
    title: tl.title,
    exid: tl.exid,
    table: "timelapses",
    requested_by: u.email,
    status: tl.status,
    camera_id: c.exid,
    created_at: tl.inserted_at
  })

Archive
|> join(:left, [a], u in assoc(a, :user))
|> join(:left, [a], c in assoc(a, :camera))
|> select([a, u, c], %{
  title: a.title,
  exid: a.exid,
  table: "archives",
  requested_by: u.email,
  status: a.status,
  camera_id: c.exid,
  created_at: a.created_at
})
|> union_all(^compares)
|> union_all(^timelapses)

这给了我所有的结果，但是由于数据很多，我想对其进行分页，我尝试使用scrivener_ecto，但由于出现这样的错误而失败

* (Postgrex.Error) ERROR 42601 (syntax_error) each UNION query must have the same number of columns 

    query: SELECT count('*') FROM "archives" AS a0 LEFT OUTER JOIN "users" AS u1 ON u1."id" = a0."requested_by" LEFT OUTER JOIN "cameras" AS c2 ON c2."id" = a0."camera_id" UNION ALL (SELECT c0."name", c0."exid", 'compares', u1."email", c0."status", c2."exid", c0."inserted_at" FROM "compares" AS c0 LEFT OUTER JOIN "users" AS u1 ON u1."id" = c0."requested_by" LEFT OUTER JOIN "cameras" AS c2 ON c2."id" = c0."camera_id") UNION ALL (SELECT t0."title", t0."exid", 'timelapses', u1."email", t0."status", c2."exid", t0."inserted_at" FROM "timelapses" AS t0 LEFT OUTER JOIN "users" AS u1 ON u1."id" = t0."user_id" LEFT OUTER JOIN "cameras" AS c2 ON c2."id" = t0."camera_id")
    (ecto_sql 3.6.1) lib/ecto/adapters/sql.ex:749: Ecto.A

是否有可能对union_all 查询应用分页以获得此类数据

%{
  from: 1,
  items: [],
  limit: 50,
  page: 1,
  to: 50,
  total: 3618
}

任何帮助将不胜感激，如果您在查询中看到任何令人担忧的内容或可以更改为更好的内容，请同时提出建议。谢谢。

Answer 1

问题在于这个查询：

SELECT count('*') FROM "archives" AS a0 LEFT OUTER JOIN "users" AS u1 ON u1."id" = a0."requested_by" LEFT OUTER JOIN "cameras" AS c2 ON c2."id" = a0."camera_id" UNION ALL (SELECT c0."name", c0."exid", 'compares', u1."email", c0."status", c2."exid", c0."inserted_at" FROM "比较" AS c0 左外连接 "用户" AS u1 ON u1."id" = c0."requested_by" 左外连接 "cameras" AS c2 ON c2."id" = c0."camera_id") UNION ALL (SELECT t0."title", t0."exid", 'timelapses', u1."email", t0."status", c2."exid", t0."inserted_at" FROM "timelapses" AS t0 LEFT OUTER JOIN "users " AS u1 ON u1."id" = t0."user_id" LEFT OUTER JOIN "cameras" AS c2 ON c2."id" = t0."camera_id")

如您所见，第一个查询正在尝试获取计数，而最后两个查询正在尝试获取字段，并且由 UNION ALL 子句连接的查询选择的字段不匹配。形成的查询不正确。

如果您使用的是scrivener_ecto 库，问题肯定出在其中。

为了更清楚地说明这一点，大多数分页库通过了解从数据库中获取的确切行数来计算previous_page、@ 987654324@等要在API中返回。

我不知道有任何 elixir 库可以处理这个问题，但这可以通过编写自定义查询来实现：

Archive
|> join(:left, [a], u in assoc(a, :user))
|> join(:left, [a], c in assoc(a, :camera))
|> select([a, u, c], %{
  title: a.title,
  exid: a.exid,
  table: "archives",
  requested_by: u.email,
  status: a.status,
  camera_id: c.exid,
  created_at: a.created_at
})
|> union_all(^compares)
|> union_all(^timelapses)
|> order_by(:created_at)
|> limit(50)
|> offset((page-1)*50)

您将不得不期待一个额外的参数page。 您可以编写另一个查询来获取 total 参数的所有此类记录的计数。

P.S.：除非确实需要，否则我会避免返回 total 参数的行数。当表中的行数增加时，使用如此多的连接和联合支持 count(*) 肯定会给您带来很多性能问题。