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Re: [freetds] dbnextrow() question

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  • From: craigs <s.craig AT andronics.com>
  • To: brian AT bruns.com, FreeTDS Development Group <freetds AT lists.ibiblio.org>
  • Cc:
  • Subject: Re: [freetds] dbnextrow() question
  • Date: Mon, 14 Mar 2005 15:13:13 +0000
Bill,

Could you have a look at my first attempt, first time writing stored procedures/functions and im sure i've got something wrong.

CREATE PROC GetPostcodeRecord ( @my_lat float, @my_long float )
AS
BEGIN
SELECT TOP 1 PlaceName, Town, Country, Latitude, Longitude
FROM PostCodes PC
WHERE LatLonDistance( @my_lat, @my_lon, PC.Latitude, PC.Longitude ) < 1
END

**************************************************************************************************************************************

CREATE FUNCTION LatLonDistance(@latitude float, @longitude float, @prev_lat float, @prev_lon float)
RETURNS float AS
BEGIN
declare @radiusEarth integer
declare @delta_lat float
declare @delta_lon float
declare @intermed_a float
declare @intermed_c float
declare @result float
set @radiusEarth = 6367
@delta_lon = AsRadians(@prev_lon) - AsRadians(@longitude);
@delta_lat = AsRadians(@prev_lat) - AsRadians(@latitude) ;
@intermed_a = Sin2(@delta_lat / 2) + cos( AsRadians( @latitude )) * cos( AsRadians( @prev_lat )) * Sin2( @delta_lon / 2);

@intermed_c = 2 * Arcsin( GetMin( 1, sqrt( @intermed_a )));

@result = @radiusEarth * @intermed_c;

return @result;
END

**********************************************************

CREATE FUNCTION GetMin(@x float, @y float)
RETURNS float AS
BEGIN
if ( @x <= @y )
return @x;
else
return @y;
END

**********************************************************

CREATE FUNCTION AsRadians( @x float )
RETURNS float AS
BEGIN
declare @my_pi float
declare @result float
set @my_pi = 3.14159265358979
@result = @x * ( @my_pi / 180 )
return @result
END

**********************************************************

CREATE FUNCTION Sin2( @x float )
RETURNS float AS
BEGIN
declare @result float
@result = (1 - cos(2 * @x)) / 2;
return @result;
END

**********************************************************

CREATE FUNCTION Arcsin( @x float )
RETURNS float AS
BEGIN
declare @result float
@result = atan( @x / sqrt( -@x * @x + 1));
return @result;
END
**********************************************************

Brian Bruns wrote:

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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