MySQL query never end
You have UNIQUE(end_time, instr), correct?  Then the fast query (SELECT * FROM cs.cs_15 where instr like '%X_T%' and end_time&gt;'2016-09-22' LIMIT 0, 1000) will be performed this way.  (Confirm it by doing EXPLAIN SELECT ....)


Reach into that index at '2016-09-22'.
Scan forward, looking for instr like '%X_T%'
peel off the first 1000 that match.


Note: Apparently many of the rows have X_T, so the query ends quickly.

The slow query (SELECT * FROM cs.cs_15 where instr like '%X_T%' limit 1) will be performed this way.  (Confirm it by doing EXPLAIN SELECT ....)


No index is useful, so scan the entire table, looking for 1 row with instr like '%X_T%'.


Note: Apparently there is no such row early in the table.  That is, old end_times never had X_T.

There is not good way to speed up the slow query.  The leading wildcard in the LIKE make it useless to index that column.