What is the algorithmic complexity of a PosgtreSQL greater than or lesser than index query (compared to equality)?
I don't know the PostgreSQL engine, however in the real world, doing 

WHERE X &gt; y

will cause the engine to seek to the point in the B-tree index where X = y, then scan the rest of the data to return the relevant rows.  With enough data, it will take significant time to return.

Clearly, WHERE X = y will return more quickly since it only has to return a single row, necessitating only the initial seek operation.

This is, of course, assuming X is indexed.