Application of MySQL prefix index

In MySQL, sometimes the columns that need to be indexed are very long, and if you apply the index directly it can cause problems with indexing too large. So we can take some of the fields to do the indexing, for example:

Add index: ALTER TABLE * Add key (field (3)); field index fields, number of 3 index characters

But how do we determine how long the number of characters to take the most reasonable index length:

By: Select count (Field)/count (*) from table, statement gets selection rate,

Then: SELECT COUNT (distinct left (field, N))/count (*) from table

Use this statement to fit the most reasonable n.

Application of MySQL prefix index