Af far back as the sixties, the indexes of file and database systems were built as balanced trees.
Index rows are stored in leaf pages. Today, the typical index page size is 4 or 8 kb. The lenght of an index row is usually the sum of the length of the index columns plus about 10 bytes of control information. If a total length of an index row is 200 bytes, about 40 index rows will fit in an 8K leaf page if the index is unique.
In a nonunique index, several pointers may follow each key value; in many DBMS these pointers are stored in pointer value sequence. This is enable the DBMS to quickly find a pointer to be deleted even if there are a million pointers chained to one key value. We mention this because for historical reasons, some DBAs are worried about the impact of the maximum chain length on delete performance.
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