Re: why do we need create tuplestore for each fetch?

高增琦 <pgf00a@gmail.com> writes:
> 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.

How are you trying to do the fetches, PQexec("fetch 3 in c") ?
That is an inherently inefficient way to do things, and trying to shave
a few cycles off the intermediate tuplestore isn't going to fix that.
The general overhead of parsing a new SQL command is probably going to
swamp the costs of a tuplestore, especially if it's too small to spill
to disk (and if it isn't, you really do need the tuplestore mechanism,
slow or not).

If you want to get a speed improvement there would probably be a lot
more bang for the buck in extending libpq to support protocol-level
portal access.  It does already have PQdescribePortal, but for some
reason not anything for "fetch N rows from portal so-and-so".  Not
sure whether it's worth providing explicit portal open/close commands
separate from PQexec'ing DECLARE CURSOR and CLOSE, but maybe at the
margins those steps would be worth improving too.
        regards, tom lane