Hi Laurenz:
Thank you for your kind reply.
Please let me dig it a little more:
I think that when a session is accessing a postgersql table. It will be influenced by the followings:
Really old data (needed to be vacuumed, eg: old image at one day ago).
Recent data (committed and uncommitted), because they are all in the data block.
Isn’t it strange that I have to access my data among somebody’s un-decided data?
How if there is a busy system having a table accessed by so many sessions at same time？
They will all be slowed down because of uncommitted but flushed out data, I think.
I hope in future the architecture of PostgreSQL can put the committed data & uncommitted data apart,
Or even put them in separate physical disks.That will Help to improve performance I think.
"SELECT count(*) FROM ptest" may be simple, but it is expensive高健 wrote:
> I have one question about the visibility of explain plan.
>
> Firstly , I was inserting into data to a table. I use : [ insert into ptest select * from
> test02; ]
>
> And test02 table has 10,000,000 records. And ptest is a parent table, which has two distribution
> child table --- ctest01 and ctest02.
>
> When I execute the above sql statement, it takes some time to execute because of data volume.
>
> Before the above sql statement finish, I open another session with psql, and execute: [ select
> count(*) from ptest; ]
> Because the insert into statement in other session has not finished, I got the result of zero.
>
> Before first session finish, If I check the explain of select, I got:
> postgres=# explain select count(*) from ptest;
> QUERY PLAN
> -----------------------------------------------------------------------------------
> Aggregate (cost=55406.40..55406.41 rows=1 width=0)
> -> Append (cost=0.00..49601.92 rows=2321793 width=0)
> -> Seq Scan on ptest (cost=0.00..0.00 rows=1 width=0)
> -> Seq Scan on ctest01 ptest (cost=0.00..24776.52 rows=1159752 width=0)
> -> Seq Scan on ctest02 ptest (cost=0.00..24825.40 rows=1162040 width=0)
> (5 rows)postgres=#
>
> I think that is because postgresql is holding commited and uncommited data together
> physically(vacuum is needed because of it?).
>
> Is there some method that allow simple select such as select count(*) do not activate the explain
> plan ?
> (I think the more table is bigger, the more sample data is needed)
because it has to visit every tuple in the table.
The first time you run it after the insert it might also trigger
considerable write activity (hint bits), but that on the side.
If you need only an estimate, try
SELECT sum(reltuples) FROM pg_class WHERE relname IN ('ptest', 'ctest01', 'ctest02');
Yours,
Laurenz Albe
