I've wanted for a long time to have EXPLAIN ANALYZE output per-node I/O usage.
This would be especially useful if we could distinguish hardware versus
logical I/O though. And I always thought that would be very hard.
My thought in the past was that would could do it on Solaris by having
Postgres use DTrace directly via its (undocumented but existing) programmatic
interface.
For other operating systems it was tempting to suggest just timing the read(2)
call to see if it took too long to be a logical operation. The problem there
is that gettimeofday would impose far too much overhead to make that practical
(or even be precise enough to work properly).
But it occurred to me just now that the hardware instruction counter available
on just about every platform would be good enough for a heuristic guess at
whether the read(2) was cached. The problem generally with using the hardware
instruction counter is that it's not necessarily in sync between processors
and might therefore run backwards or skip time forwards. This is a problem for
profiling but if all we care about is a boolean guess at whether the request
was satisfied quickly from cache then any such skipping forward or backward
would represent a context switch which we could just toss in the hardware i/o
bucket. It doesn't matter exactly how long the hardware i/o took, only that
there was one.
To that end I would love to see something like:
QUERY PLAN
-------------------------------------------------------------------------------------------------------------Bitmap
HeapScan on h (cost=8.52..16.45 rows=2 width=512) (actual time=78.926..87.708 rows=2 loops=1 logical-I/O=2
physical-I/O=1) Recheck Cond: (i = ANY ('{100,1000}'::integer[])) -> Bitmap Index Scan on hi (cost=0.00..8.52 rows=2
width=0)(actual time=74.539..74.539 rows=2 loops=1 logical-I/O=2 physical-I/O=2)) Index Cond: (i = ANY
('{100,1000}'::integer[]))Totalruntime: 87.820 ms
-- Gregory Stark EnterpriseDB http://www.enterprisedb.com Ask me about EnterpriseDB's PostGIS support!
