Thanks for you reply.
I found query without cursor is faster then query with server-side cursor and several fetches.
But I have a large result set to retrieve from database. I have to choose server-side cursor
to avoid out-of-memory problem.
When I try to debug the cursor and fetch, I found this unexpected behavior. I think maybe
the tuplestore slows the cursor. (maybe I should do some profile later)
I want to change the code, but I am afraid there are important reasons for the tuplestore.
Therefore, I post it to this list for help: why create tuplestore for each fetch?
p.s. a large fetch may turn tuplestore to use buffer file, and slow the performance very much.
On Mon, Dec 19, 2011 at 9:06 PM, Robert Haas
<robertmhaas@gmail.com> wrote:
On Thu, Dec 15, 2011 at 8:30 AM, 高增琦 <
pgf00a@gmail.com> wrote:
> I found this several days ago when I try to debug a "fetch" of cursor.
> And I have sent a mail to this list, but no one reply...
> Maybe this is a very simple problem, please help me, thanks a lot...
>
> Here is the example:
> create table t (a int);
> insert into t values (1),(3),(5),(7),(9);
> insert into t select a+1 from t;
> begin;
> declare c cursor for select * from t order by a;
> fetch 3 in c;
> fetch 3 in c;
> fetch 3 in c;
>
> In 'PortalRun', a fetch stmt will be treated with PORTAL_UTIL_SELECT,
> and then a tuplestore will be created in 'FillPortalStore' in the
> fetch stmt's portal.
>
> In 'FillPortalStore', all result will be store at that tuplestore,
> Then, go back to 'PortalRun'; next, 'PortalRunSelect' will send this
> results to client...
>
> My problem is: why do we need create that tuplestore as an
> middle storeage? why do not we just send these result to clent
> at the first time?
three-row fetch, but maybe it does for larger ones? What is your
motivation for investigating this?
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
--
GaoZengqi
pgf00a@gmail.comzengqigao@gmail.com
