Why most SQL databases allow defining the same index twice?

Question

Why most SQL databases allow defining the same index (or constraint) twice?

For example in MySQL I can do:

CREATE TABLE testkey(id VARCHAR(10) NOT NULL,          


        

Answer 1

Because databases that support covering indexes - Oracle, MySQL, SQL Server... (but not PostgreSQL, oddly). A covering index means indexing two or more columns, and are processed left to right for that column list in order to use them.

So if I define a covering index on columns 1, 2 and 3 - my queries need to use, at a minimum, column 1 to use the index. The next possible combination is column 1 & 2, and finally 1,2 and 3.

So what about my queries that only use column 3? Without the other two columns, the covering index can't be used. It's the same issue for only column 2 use... Either case, that's a situation where I would consider separate indexes on columns 2 and 3.