PostgreSQL触发器动态生成多个表的代码

Question

我想为数据库中的许多表生成代码，在我准备编写第三个“获取表X代码”的实现时停下来重构我的解决方案。

我的代码如下：

-- Tenants receive a code that's composed of a portion of their subdomain and a unique number.
-- This number comes from this sequence.
CREATE SEQUENCE tenant_codes_seq MAXVALUE 9999 NO CYCLE;

CREATE TABLE tenants (
    subdomain   varchar(36) NOT NULL UNIQUE
  , tenant_code char(8)     NOT NULL UNIQUE
  , PRIMARY KEY (tenant_code)
);

-- This function expects four parameters:
-- 1. The column that's receiving the generated code (RECEIVING_COLUMN_NAME)
-- 2. The column that's used to salt the code (SALT_COLUMN_NAME)
-- 3. The number of characters to use from the salt column (SALT_LENGTH)
-- 4. The sequence name, but defaults to RECEIVING_COLUMN_NAME || 's'
CREATE OR REPLACE FUNCTION generate_table_code() RETURNS trigger AS $$
DECLARE
  receiving_column_name text;
  salt_column_name      text;
  salt_length           text;
  sequence_name         text;
BEGIN
  receiving_column_name := TG_ARGV[0];
  salt_column_name      := TG_ARGV[1];
  salt_length           := TG_ARGV[2];

  CASE
  WHEN TG_NARGS = 3 THEN
    sequence_name := receiving_column_name || 's';
  WHEN TG_NARGS = 4 THEN
    sequence_name := TG_ARGV[3];
  ELSE
    RAISE EXCEPTION '3 or 4 arguments expected, received %', TG_NARGS;
  END CASE;

  -- The intent is to return ABC-0001 when salt_column contains 'ABC'
  EXECUTE 'rpad(substr('                ||
          quote_ident(salt_column_name) ||
          ', 1, 4), 4, '                ||
          quote_literal('-')            ||
          ') || lpad(nextval('          ||
          quote_literal(sequence_name)  ||
          ')::text, '                   ||
          quote_literal(salt_length)    ||
          ', '                          ||
          quote_literal('0')            ||
          ')'
  INTO STRICT NEW;
  RETURN NEW;
END
$$ LANGUAGE plpgsql;

CREATE TRIGGER generate_tenant_code_trig
  BEFORE INSERT ON tenants FOR EACH ROW
  EXECUTE PROCEDURE generate_table_code('tenant_code', 'subdomain', 4);

如何将值分配给NEW.tenant_code、NEW.user_code或NEW.table_whatever_code？

运行一些测试可以得到正确的“语句”，但我似乎无法正确地进行赋值：

INSERT INTO tenants(subdomain) VALUES ('abc')


CREATE TABLE
ERROR:  syntax error at or near "NEW"
LINE 1: NEW.tenant_code := rpad(substr(subdomain, 1, 4), 4, '-') || ...
        ^
QUERY:  NEW.tenant_code := rpad(substr(subdomain, 1, 4), 4, '-') || lpad(nextval('tenant_codes')::text, '4', '0'::text)
CONTEXT:  PL/pgSQL function "generate_table_code" line 20 at EXECUTE statement

- François Beausoleil

Answer 1

我很乐意被证明是错误的（有时我自己也需要这样），但据我所知，使用变量引用列名是一种情况，您实际上需要使用PL/C触发器而不是PL/PgSQL触发器。在contrib/spi和PGXN上可以找到此类触发器的示例。

或者，将列命名一致，以便能够直接引用它们，例如NEW.tenant_code。

个人而言，我通常会编写一个创建触发器的函数：

create function create_tg_stuff(_table regclass, _args[] text[])
  returns void as $$
begin
  -- explore pg_catalog a bit
  execute $x$
  create function $x$ || quote_ident(_table || '_tg_stuff') || $x$()
    returns trigger as $t$
  begin
    -- more stuff
    return new;
  end;
  $t$ language plpgsql;
  $x$;
end;
$$ language plpgsql;

- Denis de Bernardy

---

谢谢，那就是我要做的事情，只不过是用 Ruby / ERB 而不是 PostgreSQL 本身。 - François Beausoleil

Answer 2

NEW 是类型 RECORD，所以据我所知你不能对其进行赋值。

要设置列的值，请将其分配给 NEW.column，例如：

NEW.tenant_code := (SELECT some_calculation);

也许你的设计过于复杂；PL/SQL 是一种非常有限的语言 - 尽可能使你的代码简单。

- Bohemian