MySQL???????????????

?????????????? id?4???????????????????????β???У?????????????????????????5.5?汾?????????????????????????????5.6???limit M??N????????????????У?????????в??????????????????????order by c1 asc limit 0??3 ????????С?3??????limit 3??3????????С?6??????????c1?2??????3????????????????????(?????????key???????????????????????λ?????)????????·??????????????????????????????????????м??????????????id??????????id????????????????key??????????SQLд?????￡?
????select * from t1 order by c1??id asc limit 0??3;
????select * from t1 order by c1??id asc limit 3??3;
????????2
??????????????????????????в??????????????????????????????
????????????????
????create table t2(id int primary key?? status int?? c1 varchar(255)??c2 varchar(255)??c3 varchar(255)??key(c1));
????insert into t2 values(7??1??'a'??repeat('a'??255)??repeat('a'??255));
????insert into t2 values(6??2??'b'??repeat('a'??255)??repeat('a'??255));
????insert into t2 values(5??2??'c'??repeat('a'??255)??repeat('a'??255));
????insert into t2 values(4??2??'a'??repeat('a'??255)??repeat('a'??255));
????insert into t2 values(3??3??'b'??repeat('a'??255)??repeat('a'??255));
????insert into t2 values(2??4??'c'??repeat('a'??255)??repeat('a'??255));
????insert into t2 values(1??5??'a'??repeat('a'??255)??repeat('a'??255));
??????????SQL???
????select id??status??c1??c2 from t2 force index(c1) where c1>='b' order by status;
????select id??status from t2 force index(c1) where c1>='b' order by status;
??????н?????￡?

???????????????м????????

??????????????????????м???force index??hint???????????c1??????????????c1?????????id???????????????????С?????c1??????С???????c1?????е????λ?????￡?
????(c1??id)===(b??6)??(b??3)??(5??c)??(c??2)???????status?????2 3 2 4?????????????????status?????????λ????(6??2??b)??(5??2??c)??(3??3??c)??(2??4??c)?????????????????????????????????????????(6??2??b)??(5??2??c)???????????????????????????a.?????????b.????????б??????????????????????????????????????е????????????max_length_for_sort_data????????????????????????????????????MYSQL??rowid?????????IO?????IO????????????5?????6?????????????????????????????е??????????????????????????????????λ?á???????????????????????????????max_length_for_sort_data??????????????2048??