在Postgres中訂購WITH RECURSIVE查詢

Question

我在Postgres中執行遞歸查詢以檢索電子郵件及其線程子項列表，如下所示：

WITH RECURSIVE cte (id, title, path, parent_id, depth)  AS (
  SELECT  id, 
          title,
          array[id] AS path,
          parent_id, 
          1 AS depth
  FROM    emails
  WHERE   parent_id IS NULL

  UNION ALL

  SELECT  emails.id,
          emails.title,
          cte.path || emails.id,
          emails.parent_id, 
          cte.depth + 1 AS depth
  FROM    emails
          JOIN cte ON emails.parent_id = cte.id
)
SELECT id, title, path, parent_id, depth FROM cte
ORDER BY path;

如何在查找兒童電子郵件之前更改列表的順序（例如，對標題進行排序）。 我顯然需要保留外部ORDER BY，以便按照樹的順序檢索列表，Postgres不允許我在UNION ALL之前插入ORDER BY子句。

謝謝，

Answer 1

創建一個由查詢的第一部分組成的視圖，按標題排序。 也許是這樣的？

      CREATE VIEW title_Sort AS
      SELECT  id,  
      title, 
      array[id] AS path, 
      parent_id,  
      1 AS depth  
      FROM    emails 
      WHERE   parent_id IS NULL 
      ORDER BY title;

然后UNION ALL使用您之前執行的其他查詢查看。 我認為這會奏效。 在我的上網本上，所以我無法測試：/

Answer 2

這是未經測試的，但通常我可以在聯合之前添加任何ORDER BY，只要有括號...

WITH RECURSIVE cte (id, title, path, parent_id, depth)  AS (
(  SELECT  id, 
          title,
          array[id] AS path,
          parent_id, 
          1 AS depth
  FROM    emails
  WHERE   parent_id IS NULL
  ORDER BY title
)
  UNION ALL

  SELECT  emails.id,
          emails.title,
          cte.path || emails.id,
          emails.parent_id, 
          cte.depth + 1 AS depth
  FROM    emails
          JOIN cte ON emails.parent_id = cte.id
)
SELECT id, title, path, parent_id, depth FROM cte
ORDER BY path;