2013/12/11 Jeff Janes
> On Tuesday, December 10, 2013, jacket41142 wrote:
>
>> Thanks very much.
>>
>> I think another problem is that the cost estimation isn't good enough to
>> reflex real cost. Since we can see, from "explain analyze ...",
>> count(distinct ...) has smallest cost between the o
Hi
I'm just trying about PostgreSQL, I create a database "test" with a table
"t1":
test=> \d t1
Table "public.t1"
Column | Type | Modifiers
-+-+-
col_id | integer | not null
On Tuesday, December 10, 2013, jacket41142 wrote:
> Thanks very much.
>
> I think another problem is that the cost estimation isn't good enough to
> reflex real cost. Since we can see, from "explain analyze ...",
> count(distinct ...) has smallest cost between the others, but since it uses
> sorts
I have done another experiment for count(*) vs count(distinct ...), on same
table schema but with 1000 rows now. And for this time, the postgres
version is 9.3.2 (9.3.2-1.pgdg12.4+1).
These two has same resulted query plan with same estimated cost, but
count(*) is straightly fast.
test=> expla
Thanks very much.
I think another problem is that the cost estimation isn't good enough to
reflex real cost. Since we can see, from "explain analyze ...",
count(distinct ...) has smallest cost between the others, but since it uses
sorts, the time complexity should be higher especially for large am
On Tue, Dec 10, 2013 at 9:28 AM, jacket41142 wrote:
>
> test=> select distinct col_int from t1 group by col_int;
> Time: 1177.936 ms
>
> So the performance difference is not very large.
> But when I do that:
>
> test=> select count(distinct col_int) from t1;
> count
> ---
> 1025
> (1 row)
jacket41142 wrote:
> [ subject issue in detail ]
Please review this thread:
http://www.postgresql.org/message-id/flat/CAPNY-2Utce-c+kNTwsMCbAk58=9myeaevitxt9lo7r1k77j...@mail.gmail.com#CAPNY-2Utce-c+kNTwsMCbAk58=9myeaevitxt9lo7r1k77j...@mail.gmail.com
--
Kevin Grittner
EDB: http://www.enterpri
Hi
I'm just trying about PostgreSQL, I create a database "test" with a table
"t1":
test=> \d t1
Table "public.t1"
Column | Type | Modifiers
-+-+-
col_id | integer | not null
