fix 90452 threading by subject slow when threads are very large, sr=mscott, a=bsmedberg

mailnews/db/msgdb/src/nsMsgThread.cpp

@@ -290,6 +290,19 @@ NS_IMETHODIMP nsMsgThread::AddChild(nsIMsgDBHdr *child, nsIMsgDBHdr *inReplyTo,
   
   PRBool hdrMoved = PR_FALSE;
   nsCOMPtr <nsIMsgDBHdr> curHdr;
+
+  // This is an ugly but simple fix for a difficult problem. Basically, when we add
+  // a message to a thread, we have to run through the thread to see if the new
+  // message is a parent of an existing message in the thread, and adjust things
+  // accordingly. If you thread by subject, and you have a large folder with
+  // messages w/ all the same subject, this code can take a really long time. So the
+  // pragmatic thing is to say that for threads with more than 1000 messages, it's
+  // simply not worth dealing with the case where the parent comes in after the
+  // child. Threads with more than 1000 messages are pretty unwieldy anyway.
+  // See Bug 90452
+
+  if (numChildren < 1000)
+  {
     for (childIndex = 0; childIndex < numChildren; childIndex++)
     {
       nsMsgKey msgKey;
@@ -341,6 +354,7 @@ NS_IMETHODIMP nsMsgThread::AddChild(nsIMsgDBHdr *child, nsIMsgDBHdr *inReplyTo,
         }
       }
     }
+  }
   // If this header is not a reply to a header in the thread, and isn't a parent
   // check to see if it starts with Re: - if not, and the first header does start
   // with re, should we make this header the top level header?