FreeTDS Development Group

[Brian Bruns <brian.bruns AT gmail.com>]

Here a good introductory article

http://www.databasejournal.com/features/mssql/article.php/3348181

And a tip from me, store the lon/lat in radians in your database if
possible, it'll speed things a bit and you can use something like:

select acos(cos(@lat)*cos(@lon)*cos(t.lat)*cos(t.lon) +
cos(@lat)*sin(@lon)*cos(t.lat)*sin(t.lon) + sin(@lat)*sin(t.lat)) from
t

with or without the function.  Spatial extension for MS SQL would be
nice, but unfortunately it looks like MS opted not to put them in 2005
either.

On Fri, 11 Mar 2005 16:21:10 +0000, craigs <s.craig AT andronics.com> wrote:
> Bill,
> 
> You'd be surprised how complicated it is to find the distance between 2
> lat lons!  My LatLonDistance query uses a number of other functions to
> work it out, such as pie, cos, atan, sqrt as well as many
> multiplications and divisions.
> 
> I think i will have to create a stored procedure function to call as
> well as trying to reduce the results my query will return, it should be fun!
> 
> Is there any online resources where i can look into writing SQL
> functions that anyone knows off, save me a bit of time searching.
> 
> Thompson, Bill D (London) wrote:
> 
> >Hi Shaun
> >
> >I'm going to keep going at this because it looks interesting...
> >
> >Come on - how complicated is the math to work out the distance between
> >two lat/lon pairs ?
> >
> >If you need a function - you know you can code your own functions in
> >SQL?
> >
> >
> >so you could code a SQL function (called PROXIMITY in this case), and...
> >
> >create proc get_postcode_record ( @my_lat float, @my_long float )
> >AS
> >BEGIN
> >  SELECT TOP 1 PlaceName, Town, Country, Latitude, Longitude,
> >SweepRadius
> >  FROM  PostCodes PC
> >  WHERE PROXIMITY(PC.Latitude, PC.Longitude, @my_lat, @my_lon) <
> >PC.SweepRadius
> >END
> >
> >
> >HTH,
> >
> >Bill
> >
> >
> >
> >
> >-----Original Message-----
> >From: freetds-bounces AT lists.ibiblio.org
> >[mailto:freetds-bounces AT lists.ibiblio.org] On Behalf Of craigs
> >Sent: 11 March 2005 14:08
> >To: brian AT bruns.com; FreeTDS Development Group
> >Subject: Re: [freetds] dbnextrow() question
> >
> >
> >Thanks for all the help guys.  Here's a more detailed description of my
> >problem.
> >
> >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
> >table.
> >
> >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.
> >>
> >>Brian
> >>
> >>
> >>On Thu, 10 Mar 2005 13:39:25 -0500, Lowden, James K
> >><LowdenJK AT bernstein.com> wrote:
> >>
> >>
> >>
> >>
> >>>>From: craigs
> >>>>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
> >>>someone else.
> >>>
> >>>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
> >
> >
> >>>dbrpc functions.
> >>>
> >>>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.
> >
> >
> >>>HTH.
> >>>
> >>>--jkl
> >>>
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