Re: Way to use count() and LIMIT? - Mailing list pgsql-general

Ahh...well, that is different.  What type of client are you using to connect
to the DB and get the info?

Mike

-----Original Message-----
From: Joe Koenig [mailto:joe@jwebmedia.com]
Sent: Tuesday, December 18, 2001 2:39 PM
To: SHELTON,MICHAEL (Non-HP-Boise,ex1)
Cc: 'pgsql-general@postgresql.org'
Subject: Re: [GENERAL] Way to use count() and LIMIT?


I think I wasn't clear enough - I need total rows in the result set, not
in the table. Sorry if that wasn't clear. Thanks for the info,

Joe

"SHELTON,MICHAEL (Non-HP-Boise,ex1)" wrote:
>
> I'd be surprised if a cursor is the solution you want.  A cursor is good
if
> you want to "scan" through the rows one at a time via SQL rather than a
> client software program (e.g. PHP with ADOdb or something).  If you wrote
a
> function for your DB and needed to access data from a result of a query
one
> row at a time (for calculating running totals or something -- bad example,
> but the point is the calculations need to be done _on_ the DB not the
> client) then a cursor is the way to go.
>
> Another thing about cursors is that they tie up the DB resources while
they
> are open.  And since I imagine you are wanting to have the client indicate
> when they want to scroll through the next 20 (another app/web request)
that
> couldn't be done "inside" the DB with a cursor.
>
> By the way, even if you did use the cursor you'd still need to query the
> table first for the total count before accessing 20 rows at a time, so
> that's a moot point.
>
> Also, I'd be surprised if requesting the total count of rows in a table
were
> really a hit at all (especially with no WHERE clause -- in that case the
> count is probably stored somewhere anyway and won't need to be calculated
> dynamically, again, just a guess, but probably true).
>
> Hope that helps,
>
> Mike
>
> -----Original Message-----
> From: Joe Koenig [mailto:joe@jwebmedia.com]
> Sent: Tuesday, December 18, 2001 2:29 PM
> To: SHELTON,MICHAEL (Non-HP-Boise,ex1)
> Cc: 'pgsql-general@postgresql.org'
> Subject: Re: [GENERAL] Way to use count() and LIMIT?
>
> I was currently using a LIMIT and OFFSET to move through 20 at a time. I
> need to know the total for 2 reasons:
>
> 1) To display it to the user
> 2) So my script knows whether or not to put a next button.
>
> I was hoping I could avoid 2 queries. Is the best way to do this to just
> use LIMIT and OFFSET in one query and just do a count() in the first?
> Does using a cursor offer any benefit over the LIMIT and OFFSET method?
> Thanks,
>
> Joe
>
> "SHELTON,MICHAEL (Non-HP-Boise,ex1)" wrote:
> >
> > You will also need to do a select first to get the total count.  You can
> > store it in a var then pass it back to the user for each 20 or whatever
> > amount (so each time they know total) or pass it once, then create
cursor.
> >
> > You can also use LIMIT with OFFSET to do a simple select each time for
20
> at
> > a time.
> >
> > -----Original Message-----
> > From: Jason Earl [mailto:jason.earl@simplot.com]
> > Sent: Tuesday, December 18, 2001 12:27 PM
> > To: joe@jwebmedia.com
> > Cc: pgsql-general@postgresql.org
> > Subject: Re: [GENERAL] Way to use count() and LIMIT?
> >
> > Sure, just declare a cursor.  Here's a simple one that I use:
> >
> >     DECLARE raw_data CURSOR FOR
> >     SELECT
> >         (SELECT pc FROM curpack1 WHERE curpack1.dt <= caseweights1.dt
> >          ORDER BY curpack1.dt DESC LIMIT 1) AS "pc",
> >         dt::date AS "date",
> >         dt::time AS "time",
> >         weight AS "weight"
> >         FROM caseweights1
> >         WHERE dt >= '%s' AND
> >         dt < '%s'
> >         ORDER BY dt;
> >
> > Then you simply fetch from this cursor (like so):
> >
> >     FETCH FORWARD 20 IN raw_data;
> >
> > And you close it with a simple:
> >
> >     CLOSE raw_data;
> >
> > Jason
> >
> > Joe Koenig <joe@jwebmedia.com> writes:
> >
> > > Is there a way to structure a query so you can only run 1 query, get
the
> > > full number of rows that would be returned, but then use LIMIT to step
> > > through in groups of 20? For example, a search in my CD's/Rock section
> > > will return 53,000 results. I want to give the user the number of
total
> > > results, but also use LIMIT to go through 20 at a time? Does this
> > > require 2 queries? Thanks,
> > >
> > > Joe
> > >
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