参考文献来自https://www.cnblogs.com/morewindows0/p/9223671.html
下面为自己做的测试，记下来以防忘记
EXPLAIN select * from suoyin_test where c2=‘a1‘
EXPLAIN select * from suoyin_test where c2=‘a1‘ and c1=‘a1‘
EXPLAIN select * from suoyin_test where c1 like ‘a1%‘ and c2=‘a1‘
EXPLAIN select * from suoyin_test where c1 like ‘a1%‘ and c3=‘a1‘
EXPLAIN select * from suoyin_test where c1=‘a1%‘ and c2=‘a1‘ ORDER BY c3
EXPLAIN select * from suoyin_test where c1=‘a1%‘ and c3=‘a1‘ ORDER BY c2
