Mysql covering vs composite vs column index

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.

Answer 2

This is a question I hear a lot and there is a lot of confusion around the issues due to:

• The differences in mySQL over the years. Indexes and multiple index support changed over the years (towards being supported)

• the InnoDB / myISAM differences There are some key differences (below) but I do not believe multiple indexes are one of them

MyISAM is older but proven. Data in MyISAM tables is split between three different files for:- table format, data, and indexes.
InnoDB is relatively newer than MyISAM and is transaction safe. InnoDB also provides row-locking as opposed to table-locking which increases multi-user concurrency and performance. InnoDB also has foreign-key constraints.
Because of its row-locking feature InnoDB is well suited to high load environments.

To be sure about things, make sure to use explain_plan to analyze the query execution.

Answer 3

Compound index is not the same as a composite index.

• Composite index covers all the columns in your filter, join and select criteria. All of these columns are stored on all of the index pages accordingly throughout the index B-tree.
• Compound index covers all the filter and join key columns in the B-tree, but keeps the select columns only on the leaf pages as they will not be searched, rather only extracted! This saves space and consequently creates less index pages, hence faster I/O.

Answer 4

I upvoted Johan's answer for completeness, but I think the following statement he makes regarding secondary indexes is incorrect and/or confusing;

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.

While I agree the primary key is INCLUDED in the secondary index, I do not agree MySQL "will always use a covering index" in the SELECT query specified here.

To see why, note that a full index "scan" is always required in this case. This is not the same as a "seek" operation, but is instead a 100% scan of the secondary index contents. This is due to the fact the secondary index is not ordered by the primary key; it is ordered by "indexed_field" (otherwise it would not be much use as an index!).

In light of this latter fact, there will be cases where it is more efficient to "seek" the primary key, and then extract indexed_field "from the actual table," not from the secondary index.