PostgreSQL 参数化查询

Question

我经常需要提交几乎相同的查询，但参数不同（单个参数，但在SELECT … 子句中多次（不是WHERE(!)））。

为了说明，它在duc.opt_level >= 5 /*v_min_level*/ 出现在下面的每个地方：

SELECT duc.id_du, duc.du_type_cd, duc.du_name, duc.du_addr, duc.id_du_def, duc.def_repeat_flg, defc.allows_txt_flg
  ,r.lvl + CASE duc.du_type_cd WHEN 'e' THEN CASE WHEN duc.opt_level >= 5 /*v_min_level*/ AND duc.max_occurs=1 THEN 0 ELSE 1 END ELSE 0 END lvl
  ,CASE duc.du_type_cd WHEN 'e' THEN CASE WHEN duc.opt_level >= 5 /*v_min_level*/ AND duc.max_occurs=1 THEN r.id_du ELSE duc.id_du END ELSE r.id_du END entid
  ,CASE duc.du_type_cd WHEN 'e' THEN CASE WHEN duc.opt_level >= 5 /*v_min_level*/ AND duc.max_occurs=1 THEN r.entraw ELSE duc.du_addr END ELSE r.entraw END entraw
  ,CASE duc.du_type_cd WHEN 'e' THEN CASE WHEN duc.opt_level >= 5 /*v_min_level*/ AND duc.max_occurs=1 THEN r.entnm ELSE duc.du_name END ELSE r.entnm END entnm
  ,CASE duc.du_type_cd WHEN 'e' THEN CASE WHEN duc.opt_level >= 5 /*v_min_level*/ AND duc.max_occurs=1 THEN r.pentid ELSE r.entid END ELSE r.pentid END pentid
  ,CASE duc.du_type_cd WHEN 'e' THEN CASE WHEN duc.opt_level >= 5 /*v_min_level*/ AND duc.max_occurs=1 THEN r.pentraw ELSE r.entraw END ELSE r.pentraw END pentraw
  ,CASE duc.du_type_cd WHEN 'e' THEN CASE WHEN duc.opt_level >= 5 /*v_min_level*/ AND duc.max_occurs=1 THEN r.pentnm ELSE r.entnm END ELSE r.pentnm END pentnm
  ,defc.def_type, defc.val_type
  ,defc.is_nillable
  ,defc.optional
FROM r
JOIN data_unit duc
  ON duc.id_parent_du = r.id_du
JOIN du_def defc
  ON defc.id_du_def = duc.id_du_def

…

Postgres 版本。 9.5.1

有什么优雅的方法可以参数化这个查询，这样我就可以只通过重新定义这个参数来调用它？

我想到了类似于 匿名 PL/pgSQL 块的东西，但我不知道具体是怎么回事

Answer 1

您可以交叉连接到“值”表：

SELECT duc.id_du, duc.du_type_cd, duc.du_name, duc.du_addr, duc.id_du_def, duc.def_repeat_flg, defc.allows_txt_flg
      ,r.lvl +  
         CASE duc.du_type_cd WHEN 'e' THEN CASE WHEN duc.opt_level >= p.min_level AND duc.max_occurs=1 THEN 0 ELSE 1 END ELSE 0 END lvl
      ,CASE duc.du_type_cd WHEN 'e' THEN CASE WHEN duc.opt_level >= p.min_level AND duc.max_occurs=1 THEN r.id_du ELSE duc.id_du END ELSE r.id_du END entid
      ,CASE duc.du_type_cd WHEN 'e' THEN CASE WHEN duc.opt_level >= p.min_level AND duc.max_occurs=1 THEN r.entraw ELSE duc.du_addr END ELSE r.entraw END entraw
      ,CASE duc.du_type_cd WHEN 'e' THEN CASE WHEN duc.opt_level >= p.min_level AND duc.max_occurs=1 THEN r.entnm ELSE duc.du_name END ELSE r.entnm END entnm
      ,CASE duc.du_type_cd WHEN 'e' THEN CASE WHEN duc.opt_level >= p.min_level AND duc.max_occurs=1 THEN r.pentid ELSE r.entid END ELSE r.pentid END pentid
      ,CASE duc.du_type_cd WHEN 'e' THEN CASE WHEN duc.opt_level >= p.min_level AND duc.max_occurs=1 THEN r.pentraw ELSE r.entraw END ELSE r.pentraw END pentraw
      ,CASE duc.du_type_cd WHEN 'e' THEN CASE WHEN duc.opt_level >= p.min_level AND duc.max_occurs=1 THEN r.pentnm ELSE r.entnm END ELSE r.pentnm END pentnm
      ,defc.def_type, defc.val_type
      ,defc.is_nillable
      ,defc.optional
FROM ...
  cross join (values (5)) as p(min_level)

Answer 2

假设所有列不能为 NULL（问题中缺少信息）。

将CASE 表达式解开为：

 , CASE WHEN duc.du_type_cd = 'e' AND (duc.opt_level < 5 OR duc.max_occurs <> 1) THEN r.lvl + 1   ELSE r.lvl     END AS lvl
 , CASE WHEN duc.du_type_cd = 'e' AND (duc.opt_level < 5 OR duc.max_occurs <> 1) THEN duc.id_du   ELSE r.id_du   END AS entid
 , CASE WHEN duc.du_type_cd = 'e' AND (duc.opt_level < 5 OR duc.max_occurs <> 1) THEN duc.du_addr ELSE r.entraw  END AS entraw
  -- etc.

相同的重复条件可以集中在单个LEFT JOIN 到FROM 列表中r (r1) 的另一个实例中。如果不满足条件，则默认为 COALESCE 的替代值：

SELECT duc.id_du, duc.du_type_cd, duc.du_name, duc.du_addr, duc.id_du_def, duc.def_repeat_flg
     , defc.allows_txt_flg, defc.def_type, defc.val_type, defc.is_nillable, defc.optional
     , COALESCE(r1.lvl    , r.lvl + 1)   AS lvl
     , COALESCE(r1.id_du  , duc.id_du)   AS entid
     , COALESCE(r1.entraw , duc.du_addr) AS entraw
     , COALESCE(r1.entnm  , duc.du_name) AS entnm
     , COALESCE(r1.pentid , r.entid)     AS pentid
     , COALESCE(r1.pentraw, r.entraw)    AS pentraw
     , COALESCE(r1.pentnm , r.entnm)     AS pentnm
FROM   duc
JOIN   defc ON ???  -- missing information
JOIN   r    ON ???  -- missing information
LEFT   JOIN r1 ON duc.du_type_cd <> 'e'
               OR (duc.opt_level >= 5 /*v_min_level*/ AND duc.max_occurs = 1);

现在您只需提供一次v_min_level。而且查询要短得多。也可能快一点。

并且永远不要省略关键字AS 作为列别名。 The manual:

省略 AS 关键字
...
在FROM 项中，标准和PostgreSQL 都允许省略AS 在作为非保留关键字的别名之前。但这是不切实际的 用于输出列名，因为语法不明确。