Why is multiple independent index always better than concatenated index Index Merge: Combining Multiple Indexes 


  It is one of the most common question about indexing: is it better to
  have one index for each column or a single index for all columns of a
  where clause? The answer is very simple in most cases: one index with
  multiple columns is better—that is, a concatenated or compound index.
  “Concatenated Indexes” explains them in detail.


I can totally understand this assertion under this scenario:

TABLE( A,B,C )　
Indexing 1: Index A, Index B, Index C
Indexing 2: Index (A,B,C)
Query: JOIN ON X.B=B AND X.C=C


because under this scenario, Indexing 2 cannot used at all ( I know about the exceptions )

but under this scenario:

TABLE( A,B,C )　
Indexing 1: Index A, Index B, Index C
Indexing 2: Index (A,B,C)
Query: JOIN ON X.A=A X.B=B AND X.C=C


I think Indexing 2 is better, it only needs to traverse 1 index tree instead of 3 right ?
Original author here.

The quote is meant as following:

In your first query, just filtering on B and C, what would be better:

Indexing 1: Index B, Index C
Indexing 2: Index (B,C)


Clearly, Indexing 2 is better.

The context was meant to be: if I index all columns, is it better to have one index with all of them, or one separate index for every column. In that case, it is almost always better to have one with all columns.

It's a design advice, not for assessment.