Question

I’m looking to do a SELECT with an ORDER BY on a VARCHAR(200) field. Reading the DB2 v9.1 for zOS site on this it says

Ordering is performed in accordance
with the comparison rules described in
Language elements.

And in the rules for Character string comparisons says

Two strings are compared by comparing
the corresponding bytes of each
string. If the strings do not have the
same length, the comparison is made
with a temporary copy of the shorter
string that has been padded on the
right with blanks so that it has the
same length as the other string.

My question is whether this means my ORDER BY clause will be slow as the amount of data increases because in the background a LENGTH() function call and then a padding is being carried out on every string in order to return the results in order?

Answer 1

You should use an index on the VARCHAR field. The index is a structure to optimize this process exactly.

Those comparisons you are referring to do take place when no index is present. However, if there is an index, there’s constantly a structure maintaining the order of the table by that field, so ORDER BY does not incur significant additional overhead.