How many entries are in a node of B-Tree of a MySQL Index?

Question

This online book describes how MySQL leverages B-Trees for indexing data. Time complexity depends on the number of entries per node.

How many entries does MySQL hold in one node?

Answer 1

Rule of Thumb: 100.

This applies to InnoDB data BTrees and Index BTrees. However, it can be quite far off.

One extreme: only 1 data record in a node -- due to large row size and/or activity such as DELETEs . Another case: A table with 1 record will still occupy a full 16KB block.

Other extreme: 500 index records in a node -- due to small size of key and lack of activity that would have deleted records.