Hi:
I was wondering if there is any performance-related reason to use a
unique index versus a standard index? Is the only benefit of a unique
index that it will prevent duplicate values from being inserted into a
table unless explicitly allowed?
I have a column, colA, that I know contains only
The uniqueness constraint would only be enforced during an INSERT or an
UPDATE. If your table is read-only, declaring the index as UNIQUE will be
overkill. I can't tell you about any kind of performance hit during
reading but I try to follow the maxim don't ask for it if you won't need
it. I
A unique index can actually be faster than a regular index. The reason
is that MySQL knows that there can only be a single matching row for
each value. In particular, this allows you to get const and eq_ref
for the type in an EXPLAIN, which are two of the fastest methods of
table access.
