A Btree index sorts the keys into a binary tree and stores these keys together with the table’s ROWIDs. Btree indexes are the default index type, can be unique or non-unique, and are appropriate for medium- to high-cardinality columns—those having many distinct values.

Btree-Index ，將keys值排序形成二叉樹，儲存這些keys值及 table ROWID。一般原則是：按照keys值來排序，如果是非唯一的列值，相同的兩個key按照rowid來排序，儲存。

所有index葉子節點存有資訊 key , rowid , rowid是為了select 時候能快速定位block .

Index key值(索引列上的值)以及rowid

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