database - Setting index in MySql on a Varchar Column -

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In MySQL, I have a UNION query which takes a long time to execute. The query looks 

  (date id from date, x, y, date from date (select min (date) from table WHERE flag = 0 and id = = and between date? And? ) And id =? And flag = 0 Order ID from DESC limit 1) Union (select ID, X, Y, date from date to date (selectable MAX (date) from table where flag = 0 and id =? And between date ? And?) And id =? And flag = 0 ID command by DESC limit 1)

The question above Received the best performance, which is required to index the C column and why? 

  id - type full date - type datetime flag - type tinyint (4)

I have indexed the following 1. ID 2. Date 3. Flag


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