How does MySQL index in select

How MySQL is indexed in select
To the column (named: column_a, the vegetarian in the table is: table_a) defines an index (named: index_a);

I use the general method when I search for column column_a

Select Column_a from table_a where conditional expression;

What should I do with the index index_a?

------Solution--------------------
Without understanding the index, the purpose of creating the index is to query performance for large data volumes,

When used, it is generally the case that the query condition with column_a is a lot faster than searching without creating an index, which can be seen when a certain amount of data is available.
------Solution--------------------
Explain your SQL, you can see whether the index is used in the key field
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See what your where condition is, can
Explain select column_a from table_a where conditional expression;
See if the index is used.

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