Re: Some belated patch review for "Buffers" explain analyze patch

On Wed, Feb 10, 2010 at 12:32 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> The reason that EXPLAIN prints things the way it does is so that actual
> costs/times are comparable to estimated costs.

Oh, that was a thought I had along the way but forgot to mention in my
email: since the buffer usage isn't related to the cost there isn't
nearly the impetus to divide by loops except to be consistent with the
time.

Another point is that changing the actual times to report total times
doesn't actually make much sense either. Total time to produce the
*first* record is pretty meaningless for example.

Perhaps instead of looking to change the "actual" times we should look
at a way to include total time spent in each node.

I had been experimenting with using getrusage to get more profiling
data. It also makes little sense to divide by loops since again it's
all stuff that makes sense to compare with outside data sources and
little sense to compare with the estimated costs. Perhaps we could add
the existing wall clock time to this (after pruning things like nivcsw
and minflt etc once we know what's not useful.)

postgres=# explain (analyze,buffers,resource) select * from i;                                                  QUERY
PLAN

-----------------------------------------------------------------------------------
-----------------------------Seq Scan on i  (cost=0.00..63344.86 rows=2399986 width=101) (actual
time=0.104..4309.997 rows=2400000 loops=1)  Buffers: shared hit=256kB read=307.1MB blocking=392kB  Resource Usage: user
time=656.042system time=3252.197 read=2.859MB
 
nvcsw=63 nivcsw=173 minflt=65Total runtime: 7881.809 ms



-- 
greg