MySQL status variable Handler_read_rnd_next is growing a lot
First of all, let us look at the definition of Handler_read_rnd_next.

According to the MySQL Documentation on Handler_read_rnd_next:


  The number of requests to read the next row in the data file. This
  value is high if you are doing a lot of table scans. Generally this
  suggests that your tables are not properly indexed or that your
  queries are not written to take advantage of the indexes you have.


Now, look at your query:

select data from test where data = 'vwx';


You said that the table has 10 rows. As a rule of thumb, the MySQL Query Optimizer will dismiss the use of an index if the number of rows that needs to be examined is greater that 5% of the total number of rows.

Let us do the math. 5% of 10 rows is 0.5 rows. Even if the number of rows need to locate your data is 1, that is greater than 0.5. Based of this lower number of rows and that index rule I just mentioned, MySQL Query Optimizer will always do a table scan.

Since the column data is itself indexed, instead of a table scan, mysql was performed an index scan.

If you know for a certainty that the test table will never grow, you can remove all indexes and let table scans happen. Handler status variables should stop incrementing.