享一个plpgsql中一个语句不同写法对于性能严重影响的问题(目前还找不到函数到底是怎样个调用法)

最近在测试cpu的avx指令集时无意中发现一个因为语句写法不同，性能相差极大的问题

一、定义两个测试函数

create or replace function md5_js()  returns text  as
$$
declare
   v_i integer;  
   v_md5 varchar; 
begin
   for v_i in 1..10000000 loop
       v_md5:=md5(random()::text);
   end loop;  
   return 'ok';
end;
$$
language plpgsql;

create or replace function md5_js_select()  returns text  as
$$
declare
   v_i integer;  
   v_md5 varchar; 
begin
   for v_i in 1..10000000 loop
      select md5(random()::text) into  v_md5;
   end loop;
   return 'ok';
end;
$$
language plpgsql;

[postgres@pgsqldb-master bin]$ psql
psql (9.2.3)
Type "help" for help.

postgres=# create or replace function md5_js()  returns text  as
postgres-# $$
postgres$# declare
postgres$#    v_i integer;
postgres$#    v_md5 varchar;
postgres$# begin
postgres$#    for v_i in 1..10000000 loop
postgres$#        v_md5:=md5(random()::text);
postgres$#    end loop;
postgres$#    return 'ok';
postgres$# end;
postgres$# $$
postgres-# language plpgsql;
CREATE FUNCTION
postgres=# create or replace function md5_js_select()  returns text  as
postgres-# $$
postgres$# declare
postgres$#    v_i integer;
postgres$#    v_md5 varchar;
postgres$# begin
postgres$#    for v_i in 1..10000000 loop
postgres$#       select md5(random()::text) into  v_md5;
postgres$#    end loop;
postgres$#    return 'ok';
postgres$# end;
postgres$# $$
postgres-# language plpgsql;
CREATE FUNCTION
postgres=# \timing
Timing is on.
postgres=# select md5_js();
 md5_js
--------
 ok
(1 row)

Time: 11954.169 ms
postgres=# select md5_js_select();
 md5_js_select
---------------
 ok
(1 row)

Time: 39873.332 ms
postgres=#