Mysql indexes
From wikinotes
Indexes use various datastructures to store data to prevent full scans of all rows in the table.
Index Types
BTree
A binary tree. Default for persisted storage engines.
Optimize Queries that are
- sorted
- match on field prefixes (but not suffixes)
- range bound (less than 10, between C and F)
But not queries
- match on field suffixes
Hash
A hash table. Default for the memory engine.
hash-collisions are accounted for, but each key with the same hash will need to be checked, making the query more expensive.
Indexes have a small memory footprint.Optimize queries that are
- based on the full value (ex.
IN, NOT IN, =
)But not queries
- you can't match on only a single key from a multi-key index (since both keys are hashed together)
- hash-tables are un-ordered, so range queries are not optimized (ex. less than 10, betweeen C and F)