Hi,
I have submitted a new patch against thick indexes(indexes with snapshot)
http://archives.postgresql.org/pgsql-hackers/2007-10/msg00220.php.<http://archives.postgresql.org/pgsql-hackers/2007-10/msg00220.php>
I did look closely at improving the performance of count(1) queries. It
worked well, when we are selecting a sub-set of the result-set. But when i
did a query like "select count(1) from table", it showed a improvement in
response time, but not to the extent, i wanted it to be. Let's have a look
at the stats.
gokul=# explain analyze select count(1) from dd;
LOG: EXECUTOR STATISTICS
DETAIL: ! system usage stats:
! 0.277208 elapsed 0.275457 user 0.000148 system sec
! [1.128422 user 0.004976 sys total]
! 0/0 [0/0] filesystem blocks in/out
! 0/0 [0/0] page faults/reclaims, 0 [0] swaps
! 0 [0] signals rcvd, 0/0 [6/12] messages rcvd/sent
! 0/39 [5/160] voluntary/involuntary context switches
! buffer usage stats:
! Shared blocks: 1024 Logical Reads, 0 Physical
Reads, 0 written, buffer hit rate = 100.00%
! Local blocks: 0 read, 0 written, buffer
hit rate = 0.00%
! Direct blocks: 0 read, 0 written
STATEMENT: explain analyze select count(1) from dd;
QUERY
PLAN
----------------------------------------------------------------------------------------------------------------------------------
Aggregate (cost=7323.10..7323.11 rows=1 width=0) (actual time=
276.838..276.838 rows=1 loops=1)
-> Index Only Scan using idx on dd (cost=0.00..6741.42 rows=232671
width=0) (actual time=0.042..160.753 rows=232679 loops=1)
Total runtime: 276.928 ms
(3 rows)
gokul=# set enable_indexscan=off;
SET
gokul=# explain analyze select count(1) from dd;
LOG: EXECUTOR STATISTICS
DETAIL: ! system usage stats:
! 0.331441 elapsed 0.258903 user 0.067953 system sec
! [1.906069 user 0.211479 sys total]
! 0/0 [0/0] filesystem blocks in/out
! 0/0 [0/0] page faults/reclaims, 0 [0] swaps
! 0 [0] signals rcvd, 0/0 [10/19] messages rcvd/sent
! 0/39 [9/312] voluntary/involuntary context switches
! buffer usage stats:
! Shared blocks: 5223 Logical Reads, 4391 Physical
Reads, 0 written, buffer hit rate = 15.93%
! Local blocks: 0 read, 0 written, buffer
hit rate = 0.00%
! Direct blocks: 0 read, 0 written
STATEMENT: explain analyze select count(1) from dd;
QUERY
PLAN
-----------------------------------------------------------------------------------------------------------------
Aggregate (cost=8131.39..8131.40 rows=1 width=0) (actual time=
331.075..331.076 rows=1 loops=1)
-> Seq Scan on dd (cost=0.00..7549.71 rows=232671 width=0) (actual
time=0.042..203.958 rows=232679 loops=1)
Total runtime: 331.167 ms
(3 rows)
I have run the query multiple times and it shows the response time, around
what is shown here
The table is just a multiple copy of pg_class table( approx 200000 rows). As
it can be seen, the Logical reads show a ratio of 1:5, but the response
time is not in the same ratio. I tried to profile and couldn't find anything
significant. Eventhough it shows 4391 physical reads, that's from OS cache,
since i ave already run the query multiple times.
One more disadvantage with using select count(1) using index scan is that,
it pollutes the shared memory, unlike full-table scans. But something can be
done in the regard.
Any thoughts?
Thanks,
Gokul.
