How To Create A 'Two-Sided' Unique Index On Two Fields?

Question

How can I efficiently create a unique index on two fields in a table like this: create table t (a integer, b integer);

where any unique combination of two different

Answer 1

In Oracle you could use a function-based index like this:

create unique index mytab_idx on mytab (least(a,b), greatest(a,b));

I don't know mySQL, but maybe something similar is possible? For example, you could add 2 new columns leastab and greatestab to the table, with a trigger to maintain them with the values of least(a,b) and greatest(a,b) respectively, and then create a unique index on (leastab, greatestab).