Big17,
Суммирование строк ваще тяжелая операция - зависит от способа организации строковых данных.
Это перегоняется масса байтов. Каждый раз при суммировании резервируется новый участок памяти и туда пузырятся обе строки. И если это в цикле - получается кошмар в цикле.
В VB.NET и с# для ускорения/оптимизации этой операции создан спецкласс StringBuilder.

[quot Сон Веры Павловны]Massa52Big17,
Это потому что в дотнете класс string - immutable, и конкатенация порождает новый экземпляр. В прочих случаях (например, в том же MSSQL) всё может быть совсем иначе.

http://www.sqlservercentral.com/blogs/philfactor/2009/02/16/be-careful-with-string-concatenations-in-sql-server-with-big-strings/
Posted by RBarryYoung on 18 February 2009
It has long been known that naive linear string concatenation is an O(n^2) operation (more specifically, it's triangular). That simple fact is one of the big reasons that .NET has the StringBuilder class (which uses the strings->array->mass_concatenate trick that peter alludes to).

As it happens, I spent quite a lot of time last month investigating this problem and potential solutions and let me just say: it's tough in SQL. Here are the problems:

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, it just partially alleviates it at the lower end because some percentage(k) of appends can be extensions instead of creating a new string. Effectively it changes the {(n)*(n+1)/2} cost of the naive implementation to {n +(n)*(n-1)/(2*k)} where "k" is that percentage. It's better, but it's still O(n^2).

2) The "array & mass-concatenate" trick is not available to T-SQL, not because SQL doesn't have arrays (tables serve the same purpose), but because AFAIK, there is no function that can take a variable "collection" of strings (table, array, whatever) and produce an output string.

3) The "pre-allocate and Stuff" trick popular with mutable strings is not workable in T-SQL because the STUFF() function in T_SQL is NOT like the function of the same name in some general purpose languages: the T-SQL STUFF() is an RHS (right-hand side) function and NOT an LHS (left-hand side) function. AFAIK, there is no function in SQL that can (physically) write into a pre-existing string.

That all said, I did eventually find a way to do it. Not in the O(n) time (linear) achievable in most general purpose languages, but I could get it down to O(n*Log(n)) time which is still a huge improvement. Unfortunately, I do not have it together in a presentable form and it would take me some time to do so (a day or so), but I can get it to you if you want.