Re: Thick indexes - a look at count(1) query

On Jan 16, 2008, at 8:07 AM, Gokulakannan Somasundaram wrote:
> 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.

Have you tried just executing the query with executor stats on? You
could be seeing the overhead of explain analyze...
--
Decibel!, aka Jim C. Nasby, Database Architect decibel(at)decibel(dot)org
Give your computer some brain candy! www.distributed.net Team #1828