Re: Thick indexes - a look at count(1) query - Mailing list pgsql-hackers

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@decibel.org
Give your computer some brain candy! www.distributed.net Team #1828