使用函数处理负偏移

Question

我正在创建一个 Postgresql 函数来处理负偏移量。当偏移量为负时，我的函数应该返回 0。但是，当我尝试运行此函数时，“行”附近有一个错误，我不知道为什么。我的条件分支出了什么问题？

CREATE OR REPLACE FUNCTION 
    calc_offset(row integer, padding integer) RETURNS integer AS $$
    BEGIN
        if (row-1-padding) < 0 then return 0;
        else return (row-1-padding);
    END;
$$ LANGUAGE plpgsql;

WITH all_the_trimmings AS (
    SELECT ui.id, ui.name, ui.time, 
           row_number() over(order by name asc) as rownumber
    FROM user_infos ui

  ), my_row AS (
    SELECT * FROM all_the_trimmings WHERE id=1
  ), the_slice AS (
    SELECT * FROM all_the_trimmings LIMIT 5 
       OFFSET calc_offset((SELECT rownumber FROM my_row)::int,2)
  )
SELECT * from my_row
UNION ALL
SELECT * from the_slice;

SQL fiddle for the full picture

Answer 1

您的函数calc_offset() 可以替换为这个简单的SQL 函数：

CREATE OR REPLACE FUNCTION calc_offset(p_row int, padding int)
  RETURNS int AS 'SELECT GREATEST ($1 - $2 - 1, 0)' LANGUAGE sql;

而且你根本不需要函数。只需将GREATEST 与子选择一起使用：

WITH all_the_trimmings AS (
   SELECT ui.id, ui.name, ui.time, row_number()  OVER (ORDER BY name) AS rn
   FROM   user_infos ui
   )
, my_row AS (
   SELECT * FROM all_the_trimmings WHERE id=1
   )
SELECT * FROM my_row
UNION ALL
SELECT * FROM all_the_trimmings LIMIT 5 
OFFSET GREATEST ((SELECT rn - 3 FROM my_row), 0);

或者更简单：

WITH cte AS (
   SELECT ui.id, ui.name, ui.time, row_number() OVER (ORDER BY name) AS rn
   FROM   user_infos ui
   )
SELECT * FROM cte WHERE id = 1
UNION ALL
SELECT * FROM cte LIMIT 5 
OFFSET GREATEST ((SELECT rn - 3 FROM cte WHERE id = 1), 0);

SQL Fiddle showing both.

Answer 2

row 参数是保留字。尝试用其他名称替换它。如果;

CREATE OR REPLACE FUNCTION
    calc_offset(p_row integer, padding integer) RETURNS integer AS $$
    BEGIN
        if (p_row-1-padding) < 0 then 
          return 0;
        else
          return (p_row-1-padding);
        end if;
    END;
$$ LANGUAGE plpgsql;