MySQL query consumes CPU time, help me to see how to optimize this MySQL structure

MySQL query consumes CPU time, help me to see how to optimize this MySQL structure

$SQLL = "Select * from ' shops ' WHERE ' subdomain '! = ' ORDER by RAND () LIMIT 15";

Only use this query, to subdomain query, this MySQL CPU time is very high, Sina app This wants money, do not know how to optimize can reduce MySQL CPU time, is to build an index what? What should I do? And what's the primary key? Can you optimize it?

Here is the structure

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Build an index on the subdomain field and don't know if you can use the index. Try it yourself.
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No need to add an index, you can use the field of the index to add it.
------Solution--------------------
Subdomain Index Useful

The primary key will automatically generate a unique index.
------Solution--------------------
RAND () does not use this function to randomly generate several random numbers

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