karthikeyan (6/9/2009)
1) SQL Server apparently (based on my tests) already does the "buffer-extension" trick available to mutable strings. Unfortunately, the Extension trick does NOT solve the O(n^2) problem,
If it supports the "buffer-extension" trick,then why it doesn't solve O(n^2) problem. again , i need to know exactly about O(n^2).
Because, as I already explained, it can only be used part of the time.
can you explain it with example? so that i can remember it through my career.
I already did this above, here it is again:
The "buffer-extension trick" is referring to a low-level (Assembler or C) trick that takes advantage of that fact that in most OS's strings often have large gaps of unallocated memory after them. Thus if you want to add the string B$ onto the end of A$, then normally you would have to allocate a new buffer whose length is >= Len(A$) + Len(B$), then copy all of A$ inot it, then copy B$ in after A$ and then re-point A$ to the new buff (and deallocate the old). Besides all of the memory allocation overhead, this is an O(n) operation where n = Len(A$)+Len(B$).
However, if A$ just happens to have an unused portion of memory after it whose size is >= Len(B$) then, instead you can just copy B$ into that unused memory after A$ and extend A$'s length value to inlcude the appended characters.
Please read fully what we have taken the time to write and stop making us repeat ourselves.
Does this issue solved in sql2008?
The "buffer-extension trick" can never solve this problem. It is a trick that can only be used if you are not already using the memory that is immediately above your output string. There is no way that an OS or DB environment can ever guarantee this all of the time. The only true solution is to use a different algorithm.
[font="Times New Roman"]-- RBarryYoung[/font], [font="Times New Roman"] (302)375-0451[/font] blog: MovingSQL.com, Twitter: @RBarryYoung[font="Arial Black"]
Proactive Performance Solutions, Inc. [/font][font="Verdana"] "Performance is our middle name."[/font]