[freetds] dbnextrow() question
s.craig at andronics.com
Fri Mar 11 09:08:18 EST 2005
Thanks for all the help guys. Here's a more detailed description of my
I get a current latitude, longitude value into my program, what i have
to do then is compare it with my table( called PostCodes) which has the
2.4 million records to find the record im looking for. What i mean by
this is bit hard to explain but here goes.
Table PostCodes columns include:
PlaceName, Town, Country, Latitude, Longitude, SweepRadius.
When i get my lat lon values i have to do a number of things, i call a
function called LatLonDistance which takes my current lat lon values and
works out the distance between the lat lon values in each Postcodes
record(means i have to go into each record and work out the distance),
if the distance is less than the SweepRadius this is the record im
looking for, once found i do an insert into another table and call
'return NO_MORE_RESULTS' in order to stop looping around the PostCodes
I cant see any other way to do this query because i need to call
LatLonDistance on each record until i find the one i want.
Brian Bruns wrote:
>Of course, it goes without saying that you'll eventually run into
>situtations where you can only get an approximate match of the data
>you need due to external factors (merging with external data,
>calculations that can't be done by the server), in which case you'll
>need to return the smallest set you can containing your data and do as
>the loop did in this case.
>On Thu, 10 Mar 2005 13:39:25 -0500, Lowden, James K
><LowdenJK at bernstein.com> wrote:
>>>Sent: Thursday, March 10, 2005 7:03 AM
>>>i want to exit query when i get what i want,
>>>theres over 2.4 millions records in my table so really need to get
>>>this right, can someone advise me if the following code would work.
>>Bill is helping you get your dbcancel() code correct, and maybe some
>>other aspects of your application in the bargain. I want to emphasize a
>>point he made, because it may help eventually (or sooner). You didn't
>>ask for design advice, but even it's no use to you, it might help
>>Your table has 2.4 million rows. Based on some criterion, you want to
>>pick out one and discard the others. That's the genesis of wanting to
>>understand dbcancel() etc.
>>You're not the first person to have this challenge, nor the first to try
>>to approach it this way. It won't work very well, though. You'll
>>require the server to send a great many rows over the wire that you
>>don't need, wasting server, network, and client resources. In the worst
>>case, you'll need row number 2,399,999. If every row is 100 bytes, the
>>server would find and send 240 MB for the 100 you want. On average, the
>>row you want will be midway through the result set, wasting 120 MB per
>>query. It will be roughly 1,000,000 times slower than necessary.
>>The right way to do it is to tell the server what you want: write a
>>specific query for the row you're looking for. The server will search
>>the data locally (much faster) send you the one fine row. You'll call
>>dbnextrow() exactly twice: once to get the row and once to confirm there
>>are NO_MORE_ROWS. As Bill suggested, a nice refinement is to package
>>the SQL in a stored procedure and call that with your parameter(s).
>>Send the query as "execute my_proc @my_param" or, better still, use the
>>If you formulate your query to return only what you want, the networking
>>and cancellation issues go away, and you can focus on your application.
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