The problem was caused by the following scenario:
- range optimizer picks an index IDX1 which doesn't match the ORDER BY ...
LIMIT clause.
- test_if_skip_sort_order() decides to switch to index IDX2 which matches
the ORDER BY ... LIMIT.
- it runs SQL_SELECT::test_quick_select() for the second time to produce
an quick select for IDX2.
- However, test_quick_select() would figure that full index scan on IDX1
is still cheaper (its calculations ignore the LIMIT n).
Fixed this by
- passing force_quick_range=true to test_quick_select()
- in test_quick_select, don't consider full index scans if the mentioned
parameter is true.
Numerous changes in .result files are caused by test_quick_select() being
run after "early/late NULLs filtering" feature has injected NOT NULL
condition.
Backport mysql-test/t/myisam_explain_non_select_all.test from mysql-5.6
- the .result file was modified because MariaDB choses different
query plans in a number cases. Also, we don't have some of the
"incorrect EXPLAIN output" bugs that they still have.
The .test file and includes were taken verbatim with one
exception: two tests were disabled with --disable parsing:
1. @@sql_safe_updates is not enforced EXPLAINs of multitable
updates. In MariaDB, the execution itself will produce
ER_UPDATE_WITHOUT_KEY_IN_SAFE_MODE, but EXPLAIN won't.
2. Their case #71 hits some old bug in MyISAM (See their comments
in explain_non_select.inc for details).
