Re: seqscan instead of index scan - Mailing list pgsql-performance

On Mon, 30 Aug 2004, Martin Sarsale wrote:
> "Multicolumn indexes can only be used if the clauses involving the
> indexed columns are joined with AND. For instance,
>
> SELECT name FROM test2 WHERE major = constant OR minor = constant;

You can use DeMorgan's Theorem to transform an OR clause to an AND clause.

In general:
    A OR B <=> NOT ((NOT A) AND (NOT B))

So:

> But I need something like:
>
> select * from t where c<>0 or d<>0;

    select * from t where not (c=0 and d=0);

I haven't actually tried to see if postgresql would do anything
interesting after such a transformation.