索引(a,b,c)
select * from table where a=1 and b=2 order by a; 索引排序
select * from table where a=1 and b=2 order by b; 索引排序
select * from table where a=1 and b=2 order by c; 索引排序
select * from table where a=1 order by a; 索引排序
select * from table where a=1 order by b; 索引排序
select * from table where a=1 order by c; Using filesort结论：全等情况下，
1、where a和b，abc索引排序，
2、where a，ab索引排序
select * from table where a>1 order by a; 索引排序
select * from table where a>1 order by b; Using filesort
select * from table where a>1 and b=2 order by a; 索引排序
select * from table where a>1 and b=2 order by b; 索引排序
select * from table where a>1 and b=2 order by c; Using filesort
select * from table where a>1 and b=2 and c=3 order by c; 索引排序
select * from table where a>1 and b>2 order by b; Using filesort
select * from table where a>1 and b>2 and c= 3 order by c; 索引排序
select * from table where a=1 and b>2 order by b; 索引排序
select * from table where a=1 and b>2 order by c; Using filesort
select * from table where a=1 and b>2 and c=3 order by c; 索引排序
select * from table where a=1 and b>2 and c>3 order by c; Using filesort结论：带范围的情况下，
1、order by字段必须出现在where中，除了第一次范围，order by字段必须在where中全等的情况下才能索引排序
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