more Mysql indexes--from mysql ab

When MySQL uses indexes

• Using >, >=, =, <, <=, IF NULL and BETWEEN on a key.
  
  • SELECT * FROM table_name WHERE key_part1=1 and key_part2 > 5;
    
  • SELECT * FROM table_name WHERE key_part1 IS NULL;
• When you use a LIKE that doesn't start with a wildcard.
  
  • SELECT * FROM table_name WHERE key_part1 LIKE 'jani%'
• Retrieving rows from other tables when performing joins.
  
  • SELECT * from t1,t2 where t1.col=t2.key_part
• Find the MAX() or MIN() value for a specific index.
  
  • SELECT MIN(key_part2),MAX(key_part2) FROM table_name where key_part1=10
• ORDER BY or GROUP BY on a prefix of a key.
  
  • SELECT * FROM foo ORDER BY key_part1,key_part2,key_part3
• When all columns used in the query are part of one key.
  
  • SELECT key_part3 FROM table_name WHERE key_part1=1

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When MySQL doesn't use an index

• Indexes are NOT used if MySQL can calculate that it will probably be faster to scan the whole table. For example if key_part1 is evenly distributed between 1 and 100, it's not good to use an index in the following query:
  
  • SELECT * FROM table_name where key_part1 > 1 and key_part1 < 90
• If you are using HEAP tables and you don't search on all key parts with =
• When you use ORDER BY on a HEAP table
• If you are not using the first key part
  
  • SELECT * FROM table_name WHERE key_part2=1
• If you are using LIKE that starts with a wildcard
  
  • SELECT * FROM table_name WHERE key_part1 LIKE '%jani%'
• When you search on one index and do an ORDER BY on another
  
  • SELECT * from table_name WHERE key_part1 = # ORDER BY key2