select_random_ranges: mysql mrr for secondary_ranges == 2

old complex query does not use mrr

Author: rockeet

src/lua/select_random_ranges.lua

@@ -48,9 +48,9 @@ function thread_init()
       -- MySQL doesn't yet support 'LIMIT & IN/ALL/ANY/SOME subquery,
       -- So we create an extra nested subquery
       stmt = con:prepare(string.format([[
-         SELECT count(*), sum(length(c)) FROM sbtest1 WHERE id IN
-        (SELECT * FROM (SELECT id FROM sbtest1 WHERE %s LIMIT %d) as t)]],
-         ranges, sysbench.opt.range_size))
+         SELECT length(c)
+            FROM sbtest1
+            WHERE %s LIMIT %d]], ranges, sysbench.opt.range_size))
    else
       stmt = con:prepare(string.format([[
          SELECT sum(length(c))