LP:631504 - New DS-MRR optimizations must be visible in EXPLAIN

As discussed previously, it is imperative for proper automatic testing that the new DSS-MRR-CPK optimizations announce themselves in EXPLAIN . The current "using join buffer" is too generic to be usful. Another string in the Extra column would be required, or , at the very least, some indication in EXPLAIN EXTENDED + SHOW WARNINGS. If the optimization is decided on runtime, some other method of knowing that it has triggered must be provided.

Also, modifying the optimizer variable @mrr_sort_keys does not cause any visible changes in the EXPLAIN. We have queries that are experiencing performance improvements and regressions due to the recent DS-MRR optimizations, but the EXPLAIN plan does not change even a single bit.