how B-tree indexing works in mysql

Question

When I create an index for a table in mysql, I see that the index_type is type BTREE. Now although I understand about btree(s), I do not quiet understand how i

Answer 1

The database stores the value indexed 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".

Answer 2

MySQL InnoDB is actually using B+Tree which add more features than B-Tree.

Such as:

Only leaves node has values to allow more keys in the same page node to reduce tree high which will reduce I/O count.

Add a bidirectional pointer in each leaf node to make range search faster

B-Tree

B+Tree