序列名称取自变量

Question

如何创建一个新序列，其名称来自变量？

我们来看下面的例子：

CREATE OR REPLACE FUNCTION get_value(_name_part character varying)
  RETURNS INTEGER AS
$BODY$
DECLARE
    result bigint;
    sequencename character varying(50);
BEGIN
    sequencename = CONCAT('constant_part_of_name_', _name_part);
    IF((SELECT CAST(COUNT(*) AS INTEGER) FROM pg_class
        WHERE relname LIKE sequencename) = 0)
    THEN
       CREATE SEQUENCE sequencename --here is the guy this is all about
       MINVALUE 6000000
       INCREMENT BY 1;
    END IF;
    SELECT nextval(sequencename) INTO result;
    RETURN result;
END;
$BODY$
  LANGUAGE plpgsql VOLATILE

现在，假设我想要一个 _name_part = 'Whatever' 的序列，所以我输入：

SELECT get_value('Whatever');

如果序列constant_part_of_name_Whatever 不存在，我的函数应该创建它并取一个值；如果它存在，它应该只取一个值。但是，我创建了序列constant_part_of_name_sequencename。

如何将变量的值放入序列定义中以使其工作？

Answer 1

当前接受的答案存在许多问题。最重要的是，它没有考虑到 schema。

改用：

CREATE OR REPLACE FUNCTION get_value(_name_part text)
  RETURNS bigint AS
$func$
DECLARE
   _seq text  := 'constant_part_of_name_' || _name_part;
BEGIN

CASE (SELECT c.relkind = 'S'::"char"
      FROM   pg_namespace n
      JOIN   pg_class     c ON c.relnamespace = n.oid
      WHERE  n.nspname = current_schema()  -- or provide your schema!
      AND    c.relname = _seq)
WHEN TRUE THEN           -- sequence exists
    -- do nothing
WHEN FALSE THEN          -- not a sequence
   RAISE EXCEPTION '% is not a sequence!', _seq;
ELSE                     -- sequence does not exist, name is free
   EXECUTE format('CREATE SEQUENCE %I MINVALUE 6000000 INCREMENT BY 1', _seq);
END CASE;

RETURN nextval(_seq);

END
$func$  LANGUAGE plpgsql;

SQL Fiddle.

要点

• 您的测试不必要地昂贵且不正确。您需要考虑架构。同名的序列可以存在于另一个模式中，这会使您的函数失败。
  我使用当前模式作为默认模式，因为您没有另外指定。细节：
  • How does the search_path influence identifier resolution and the "current schema"

• 您还需要注意，序列的名称与同一架构中其他对象的其他名称冲突。细节：
  • How to create sequence if not exists

• varchar(50) 因为数据类型没有意义，如果输入更长的字符串可能会导致问题。只需使用text 或varchar。

• The assignment operator in plpgsql is :=, not =.

• 您可以在声明时分配变量。更短、更便宜、更清洁。

• 您需要动态 SQL，我使用 format() 和 %I 来正确转义标识符。详情：

  • INSERT with dynamic table name in trigger function

• concat() 仅在可以涉及 NULL 值时才有用。我假设你不想通过NULL。

• VOLATILE 是默认值，因此只是噪音。

• 如果要在 NULL 输入上返回 NULL，请添加 STRICT。

Answer 2

试试这个。希望这对你有用。

CREATE OR REPLACE FUNCTION get_value(_name_part character varying) RETURNS INTEGER AS
$BODY$
DECLARE
    result bigint;
    sequencename character varying(50);
    v_sql character varying;
BEGIN
    sequencename = CONCAT('constant_part_of_name_', _name_part);
    IF((SELECT CAST(COUNT(*) AS INTEGER) FROM pg_class WHERE relname LIKE sequencename) = 0)
     THEN
     v_sql :=  'CREATE SEQUENCE '||sequencename||' 
       MINVALUE 6000000
       INCREMENT BY 1;';
       EXECUTE  v_sql;
    END IF;
    SELECT nextval(sequencename) INTO result ;
    RETURN result;
END;
$BODY$
  LANGUAGE plpgsql VOLATILE

Answer 3

CREATE OR REPLACE FUNCTION get_value(_name_part character varying) RETURNS INTEGER AS
$BODY$
DECLARE
    result bigint;
    sequencename character varying(50);
BEGIN
    sequencename = CONCAT('constant_part_of_name_', _name_part);
    IF  (select exists(SELECT relname FROM pg_class c WHERE c.relkind = 'S' and relname = ''''||sequencename||'''') = false )
    THEN
    execute 'CREATE SEQUENCE '||sequencename||'MINVALUE 6000000 INCREMENT BY 1';
    else
    END IF;
    execute 'SELECT nextval('''||sequencename||''')' INTO result;
    RETURN result;
END;
$BODY$
LANGUAGE plpgsql VOLATILE