How MySQL Uses Indexes

Some MySQL index Facts

• MySQL indexes are used to find rows with specific column values quickly.
• Without an index, MySQL must begin with the first raw and then read through entire table to find rows.
• If the table has an index for the columns, MySQL can quickly determine the position to seek into the middle of the data file without having a look at all data.
• If a table has 1000 raws, this will 100 times faster than reading sequentially.
• Most MySQL indexes are stored in B-Tree.
• The database stores the index value as a B-Tree key, and the record pointer as a B-Tree value.
• Whenever you search for a record holding a certain value of an indexed column, the engine locates the key holding this value in the B-Tree, retrieves the pointer to the record and fetches the record.
• What exactly is a “record pointer”, depends on the storage engine.
  • In MyISAM, the record pointer is an offset to the record in the MYI file.
  • In InnoDB, the record pointer is the value of the PRIMARY KEY.
• In InnoDB, the table itself is a B-Tree with a PRIMARY KEY as a B-Tree key. This is what called a “clustered index” or “index-organized table”. In this case, all other fields are stored as a B-Tree value.
• In MyISAM, the records are stored without any special order. This is called “heap storage”.
• MySQL uses indexes for these operations :
  • To find the rows matching a WHERE clause quickly.
  • To eliminate rows from consideration. If there is a choice between multiple indexes, MySQL normally uses the index that finds the smallest number of rows.
  • To retrieve rows from other tables when performing joins. MySQL can use indexes on columns more efficiently if they are declared as the same type and size.
  • To find the MIN() or MAX() value for a specific indexed column.

I found these useful information on http://dev.mysql.com/doc/refman/5.0/en/mysql-indexes.html.