Question

In the following query

SELECT  col1,col2
FROM    table1
WHERE   col3='value1'
  AND   col4='value2'

If I have 2 separate indexes one on col3 and the other on col4, Which one of them will be used in this query ?

I read somewhere that for each table in the query only one index is used. Does that mean that there is no way for the query to use both indexes ?

Secondly, If I created a composite index using both col3 and col4 together but used only col3 in the WHERE clause will that be worse for the performance?
example:

SELECT  col1,col2
FROM    table1
WHERE   col3='value1'

Lastly, Is it better to just use Covering indexes in all cases ? and does it differ between MYISAM and innodb storage engines ?

Answer 1

A covering index is not the same as a composite index.

If I have 2 separate indexes one on col3 and the other on col4, Which one of them will be used in this query ?

The index with the highest cardinality.
MySQL keeps statistics on which index has what properties.
The index that has the most discriminating power (as evident in MySQL’s statistics) will be used.

I read somewhere that for each table in the query only one index is used. Does that mean that there is no way for the query to used both indexes ?

You can use a subselect.
Or even better use a compound index that includes both col3 and col4.

Secondly, If I created a composite index using both col3 and col4 together but used only col3 in the WHERE clause will that be worse for the performance? example:

Compound index
The correct term is compound index, not composite.
Only the left-most part of the compound index will be used.
So if the index is defined as

index myindex (col3, col4)  <<-- will work with your example.
index myindex (col4, col3)  <<-- will not work. 

See: http://dev.mysql.com/doc/refman/5.0/en/multiple-column-indexes.html

Note that if you select a left-most field, you can get away with not using that part of the index in your where clause.
Imagine we have a compound index

Myindex(col1,col2)

SELECT col1 FROM table1 WHERE col2 = 200  <<-- will use index, but not efficiently
SELECT * FROM table1 where col2 = 200     <<-- will NOT use index.  

The reason this works is that the first query uses the covering index and does a scan on that.
The second query needs to access the table and for that reason scanning though the index does not make sense.
This only works in InnoDB.

What’s a covering index
A covering index refers to the case when all fields selected in a query are covered by an index, in that case InnoDB (not MyISAM) will never read the data in the table, but only use the data in the index, significantly speeding up the select.
Note that in InnoDB the primary key is included in all secondary indexes, so in a way all secondary indexes are compound indexes.
This means that if you run the following query on InnoDB:

SELECT indexed_field FROM table1 WHERE pk = something

MySQL will always use a covering index and will not access the actual table. Although it could use a covering index, it will prefer the PRIMARY KEY because it only needs to hit a single row.