CERTAIN CONDITIONS

Problem:
Optimizer chooses a wrong index to fetch the doc_id which is
present only in a full-text index

Analysis:
For the testcase presented, score can be got only when idx2 is
used. But optimizer checks that idx_1 is covering and sets
that index to be used. Since doc_id is needed to get score,
and as doc_id can only be got from idx_2, optimizer
should not set idx_1 as covering.

Solution:
This is backport from 5.7. As part of the solution a new field
is added in table structure and is set when FTS_DOC_ID field
is present. no_keyread is set to true in this case indicating
that no key can be set as covering.