Re: Weird waste time when UNION ALL to an empty result set

2011/11/10 Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>:
> Emanuel Calvo <postgres(dot)arg(at)gmail(dot)com> writes:
>> postgres=# explain (buffers true, costs true, analyze true ) (select i
>> from random_values) UNION ALL (SELECT NULL LIMIT 0);
>>                                                               QUERY PLAN
>> --------------------------------------------------------------------------------------------------------------------------------------
>>  Result  (cost=0.00..16897.02 rows=1000001 width=4) (actual
>> time=0.203..13160.797 rows=1000000 loops=1)
>>    Buffers: shared hit=608 read=6289
>>    ->  Append  (cost=0.00..16897.02 rows=1000001 width=4) (actual
>> time=0.196..7925.918 rows=1000000 loops=1)
>>          Buffers: shared hit=608 read=6289
>>          ->  Seq Scan on random_values  (cost=0.00..16897.00
>> rows=1000000 width=4) (actual time=0.190..2852.144 rows=1000000
>> loops=1)
>>                Buffers: shared hit=608 read=6289
>>          ->  Limit  (cost=0.00..0.01 rows=1 width=0) (actual
>> time=0.007..0.007 rows=0 loops=1)
>>                ->  Result  (cost=0.00..0.01 rows=1 width=0) (never executed)
>>  Total runtime: 15680.066 ms
>> (9 rows)
>
>> 10 seconds to UNION *nothing*? Is an expected behavior?
>
> I'm inclined to read this result as showing that EXPLAIN ANALYZE has
> very high per-node overhead on your machine.  That is not too uncommon
> on machines that don't have any way to read the clock without a kernel
> call.  You might try comparing straight execution times (without
> using EXPLAIN) to get a clearer idea of how much it costs to pass data
> through Append/Result.  (It's not free, no.)
>

Well, you are right. I'd executed without explain and the timing is
not highly different.

Thanks for the "EXPLAIN" !

--
--
              Emanuel Calvo
              Helpame.com