Обсуждение: assignment vs SELECT INTO

I found out today that direct assignment to a composite type is (at
least in my test) about 70% faster than setting it via SELECT INTO. That
seems like an enormous difference in speed, which I haven't really been
able to account for.

Test case:

    andrew=# \d abc
           Table "public.abc"
      Column |  Type   | Modifiers
    --------+---------+-----------
      x      | text    |
      y      | text    |
      z      | integer |
    andrew=# do $x$ declare r abc; begin for i in 1 .. 10000000 loop
    select 'a','b',i into r.x,r.y,r.z; end loop; end; $x$;
    DO
    Time: 63731.434 ms
    andrew=# do $x$ declare r abc; begin for i in 1 .. 10000000 loop r
    := ('a','b',i); end loop; end; $x$;
    DO
    Time: 18744.151 ms


Is it simply because the SELECT is in effect three assignments, so it
takes nearly 3 times as long?

cheers

andrew

Andrew Dunstan <andrew@dunslane.net> writes:
>     andrew=# do $x$ declare r abc; begin for i in 1 .. 10000000 loop
>     select 'a','b',i into r.x,r.y,r.z; end loop; end; $x$;
>     DO
>     Time: 63731.434 ms
>     andrew=# do $x$ declare r abc; begin for i in 1 .. 10000000 loop r
>     := ('a','b',i); end loop; end; $x$;
>     DO
>     Time: 18744.151 ms

> Is it simply because the SELECT is in effect three assignments, so it
> takes nearly 3 times as long?

I think it's more likely that the second example is treated as a "simple
expression" so it has less overhead than a SELECT.

            regards, tom lane

On 11/03/2014 03:24 PM, Tom Lane wrote:
> Andrew Dunstan <andrew@dunslane.net> writes:
>>      andrew=# do $x$ declare r abc; begin for i in 1 .. 10000000 loop
>>      select 'a','b',i into r.x,r.y,r.z; end loop; end; $x$;
>>      DO
>>      Time: 63731.434 ms
>>      andrew=# do $x$ declare r abc; begin for i in 1 .. 10000000 loop r
>>      := ('a','b',i); end loop; end; $x$;
>>      DO
>>      Time: 18744.151 ms
>> Is it simply because the SELECT is in effect three assignments, so it
>> takes nearly 3 times as long?
> I think it's more likely that the second example is treated as a "simple
> expression" so it has less overhead than a SELECT.
>
>


Well, I accidetally left out this case:

    andrew=# do $x$ declare r abc; begin for i in 1 .. 10000000 loop
    select row('a','b',i) into r; end loop; end; $x$;
    DO
    Time: 81919.721 ms


which is slower still.

cheers

andrew