PostgreSQL查询时将时间简化作月份的函数

PostgreSQL查询时将时间简化为月份的函数
功能：数据表tab1中有timestamp类型字段tmp1，如果统计时只用到tmp1字段的年份和月份，可以用下面的函数提高查询效率

-- Function: indexable_timestamp(timestamp without time zone, text)

-- DROP FUNCTION indexable_timestamp(timestamp without time zone, text);

CREATE OR REPLACE FUNCTION indexable_timestamp(timestamp without time zone, text)
  RETURNS text AS
$BODY$
select to_char($1,$2)
$BODY$
  LANGUAGE sql IMMUTABLE STRICT
  COST 100;
ALTER FUNCTION indexable_timestamp(timestamp without time zone, text)
  OWNER TO pgsql;


用法：select count(*) from tab1 where indexable_timestamp(tmp1,'yyyy-mm-dd')>'2013-12-01';