Title: Message
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I did
this and we got a marginal speed improvement of about 10 seconds (out of
180).
Doing
further testing I found that the .AsString takes the majority of the
time.
Stacey
Corey wrote:
----- Original Message -----
Sent: Wednesday, August 07, 2002
10:20 AM
Subject: RE: [DUG]: File Export
Speed
LRow :=
'';
for i := 0 to PQuery.FieldCount - 1
do begin
LRow := LRow +
PQuery.Fields[i].AsString + '|';
end;
LRow := LRow +
#13#10;
This string concatenation will
be very slow - it is a N squared algorithm for the number of bytes copied
ie bad news.
Not quite that
bad. AFAIK Strings allocate memory a block at a time... but it's
still pretty bad. I wrote a program that passed Strings down a
recursive tree search, then rewrote it using PChars and got a major speed
increase... run time went from an average of 2 minutes to a little over
18 seconds. Definitely worth the extra coding
effort.
Yes, it is an O(N
squared) algorithm since each string concatenation will require an extension
which means allocating a new bigger string and copying the result over.
Copying the string will take an amount of time proportional to the current
length (a function of N) and this will be done a number of times that is a
function of N. Hence, overall O(N squared) where N is Rows *
Fields.
Also, the Delphi
memory manager does not handle the continual expansion access pattern very
well and gets heavily fragmented which means even the allocation step may
well start taking time proportional to some log of N. So it might even get up
to O(N squared log N). Nasty!
Change the code to
write each string field independently (i.e. avoid concatenations) and the
algorithm will run much faster and still offer text output. Some buffering on
the TFileStream may help as well to avoid many small writes but the OS should
do a fair job of that for you.
TTFN,
Paul.
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