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select * from table FORCE  INDEX (x)
where a=1201
order by c desc limit 20

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show create table xxxx;
show index from xxxx;
explain select * from table where a=1201 order by c desc limit 20

| tb_goods |          0 | PRIMARY  |            1 | iid        | A        |    2350642 |    NULL | NULL  |      | BTREE      | NULL    |
| tb_goods |          1 | cid_total |            1 | cid        | A        |        5366 |    NULL | NULL  | 


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