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GetRows! ... Don't retrieve
data any other way!
Why LOOP and Movenext? They just
slow things down...
Most people write database retrieval code like
his:
<%
' Open Database
Do UNTIL rs.eof
city=rs("city")
st=rs("state")
zip=rs("zip")
rs.movenext
... process and format data ....
LOOP
' Close database
%>
An example of such code is at:
http://www.learnasp.com/freebook/asp/dbtable.aspx
and
http://www.learnasp.com/freebook/asp/dbsimple.aspx
If there are 700 records and 3 columns for example, we have
3,500 database read requests over the wire.
+2,100 ... Each field read is a request
+700 ..... Each .movenext is +1 request
+700 ..... Each .eof test is +1 request
====
3,500 requests
Lots of round-trips. Actually I am
fudging a little here.... the recordset object actually will maintain buffers in chunk sizes
defined by rstemp.cachesize -- setting it will determine how many rows
of data it buffers when 1 record is requested. A cachesize for example of 50 would reduce
the previous examples trips to the backend database to 14 since when you asked
for the first record you got 50. Until the MOVENEXT triggers record 51-101 being
retrieved. But those buffers, and their attendant structures ain't free either.
How about 1 transfer instead
of 3500?
A better alternative code approach is GetRows!
dim myarray
myarray=rstemp.GetRows
One transfer for seven-hundred records x 3
fields for
example. We can even close Recordset and Connection BEFORE formatting or
calculating and massaging our data.
Imagine the backend
gets to make 1
transfer into a 2,100 element array and hand
the array back
- Does not hand back an expensive recordset with
an umbilical cord/server cursor
- does not then have your app issuing a very
expensive conversation with a recordset and the ensuing baggage (hundreds of
methods, property and memory overhead, read locks for each call).
- frees up resources for other scripts
Very efficient.
Here is a real world sample:
filename=/learn/test/dbtablegetrows.asp
<%@enablesessionstate=false%>
<%response.buffer=true%>
<html><head>
<TITLE>dbtablegetrows.asp</TITLE>
</head>
<body bgcolor="#FFFFFF">
<%
' displays a database in table form via GetRows
myDSN="DSN=Student;uid=student;pwd=magic"
mySQL="select * from publishers where state='NY'"
showblank=" "
shownull="-null-"
set conntemp=server.createobject("adodb.connection")
conntemp.open myDSN
set rstemp=conntemp.execute(mySQL)
If rstemp.eof then
response.write "No records matched<br>"
response.write mySQL & "<br>So cannot make table..."
Call CloseAll
response.end
end if
response.write "<table border=""1""><tr>" & vbcrlf
'Put Headings On The Table of Field Names
for each whatever in rstemp.fields
response.write "<td><b>" & whatever.name & "</b></td>" & vbcrlf
next
response.write "</tr>" & vbcrlf
' Now lets grab all the records
alldata=rstemp.getrows
Call CloseAll
if IsArray(alldata) then
numcols=ubound(alldata,1)
numrows=ubound(alldata,2)
FOR rowcounter= 0 TO numrows
response.write "<tr>" & vbcrlf
FOR colcounter=0 to numcols
thisfield=alldata(colcounter,rowcounter)
if isnull(thisfield) then
thisfield=shownull
end if
if trim(thisfield)="" then
thisfield=showblank
end if
response.write "<td valign=top>"
response.write thisfield
response.write "</td>" & vbcrlf
NEXT
response.write "</tr>" & vbcrlf
NEXT
response.write "</table>"
End If
%>
</body></html>
<%
SUB CloseAll
rstemp.close
set rstemp=nothing
conntemp.close
set conntemp=nothing
END SUB
%>
Read:
http://www.learnasp.com/advice/roundrobin.asp
to get the full sense for the importance of closing resources early.
Get Some Real Code to Use!
If your data formatting is simple:
http://www.learnasp.com/freebook/asp/dbtablegetstring.aspx
can accomplish even faster results.
Aren't numbered arrays harder for people to
read?
Sure so I have prepared an example where we number the fields with variables and make the code easier to read for people. see:
http://www.learnasp.com/freebook/asp/dbtablegetrowsnonum.aspx
and
http://www.learnasp.com/freebook/asp/subgetrows.aspx
Old Habits Die Hard... Do you
want to be an old dog that won't learn new tricks?
The main reason that people do MOVENEXT and field
at a time because that is what they learned. But let us say 300 people hit my
main page and it read a database; well if I do my reads without getrows or
getstrings I may need a much more powerful CPU as the scripts spin, round-robin
and save their context and store their data at a low-level to serve all those
users simultaneously. On a webserver every millisecond you waste in one script
may be multiplied by tens of thousands of users (Amazon.com, Cnn.com) so it
behooves traditional programmers to bite the bullet and throw away one at a time
field reads and movenexts.
Notice how my Getrows example can close RS and CN
before processing data. Lets say do LOOP and PROCESS/FORMAT in the loop. Well,
if the reads take 1.5 seconds and the processing/formatting takes 2.5 seconds
the database is open for 4 seconds. With Getrows the same operation retrieve
might take 3/4 second and the rs and cn are closed and back in the pool for
others and the formatting is done while someone else uses the connection and
whatever READ locks placed on the data rows/chunks aren't affecting others while
formatting is occurring.
Kyle Dyer writes us... "What about
Jscript?"
And he is kind enough to supply us with the results of
his research:
strSQL = "SELECT times FROM ...";
recordSet.Open(strSQL, conn);
recordSet.GetString(2,-1,"column-delim","row-delim","null");
parameters:
1: string format: has to be 2 (must be there in JavaScript)
2: number rows: -1 gets you all of them (must be there in JavaScript)
3: column delim
4: row delim
5: nullExper
Does it matter for small amounts of data?
YES!!!!!!!!
My site has SQLserver scripts that run like
lightning. I once needed to fill a 9 item listbox from Access and got 90 sec
script timeouts with movenext. Getstring never timed out. So in a real
production situation it makes weak databases feasible and of course reduces the
load on more industrial back-ends so maybe the SQLserver doesn't need as many
indexes or RAM upgrades.
Is there any faster way to
retrieve data?
"The Worlds Fastest Listbox"
http://www.learnasp.com/freebook/asp/speedappdata.aspx
shows the only faster way to display databases -- by
using caching.
Finally I hunkered down and encapsulated the speed of
getstring, caches that can be set for durations, and the elegance of named field and
flexible formatting ala:
http://www.learnasp.com/freebook/asp/rsfast.aspx
I would argue that it is the best code samples at our entire site.
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