Sergei Petrunia ff8d4009a7 MDEV-9457: Poor query plan chosen for ORDER BY query by a recent 10.1 ...

Undo the change in test_if_skip_sort_order() that set ref_key=-1 when
a variant of index_merge is used (was made in fix for MDEV-9021).

It turned out that test_if_cheaper_ordering() call below assumes that
ref_key=-1 means "no index is used", that is, "an inefficient full table
scan is done".
This is not the same as index_merge, index_merge can actually be quite
efficient. So, ref_key=MAX_KEY denotes the fact that some index is used,
not any given index.

2016-01-24 16:59:41 +03:00

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