Question

If I create a table like so:

CREATE TABLE something (column1, column2, PRIMARY KEY (column1, column2));

Neither column1 nor column2 are unique by themselves. However, I will do most of my queries on column1.

Does the multi column primary key create an index for both columns separately? I would think that if you specify a multi column primary key it would index them together, but I really don’t know.

Would there be any performance benefit to adding a UNIQUE INDEX on column1?

Answer 1

There will probably not be a performance benefit, because queries against col1=xxx and col2=yyy would use the same index as queries like col1=zzz with no mention of col2. But my experience is only Oracle, SQL Server, Ingres, and MySQL. I don’t know for sure.