????1?? ??????
??????屑????????????????sql???????????????????????mysql????屑???????oracle??????????????????SQL?????????????
???????????????mysql??屑????????????????
????2?? ??屑??????????
????2.1 ??屑?????????????explain select …………….
??????????????????????select????explain????
????2.2 ??屑?????
????Id?????????????????????????select?????????????
????????????小??校?
??????id??????????????????????锟�?
????Select_type:???????????select???????????OR????????????????
????SIMPLE??????胁?????????????UNION
????PRIMARY?????????????魏胃?????????????????????PRIMARY
????SUBQUERY????SELECT??WHERE?斜??邪???????????????????????SUBQUERY
????DERIVED????FROM?斜??邪???????????????DERIVED????????
??????????SELECT??????UNION?????????UNION??
??????UNION??????FROM?????????校????SELECT??????????DERIVED
??????UNION?????????SELECT????????UNION RESULT
????Type?????MySQL?????????????械????????“????????”???????????????
????ALL??Full Table Scan?? MySQL???????????瑁�
????index??Full Index Scan??index??ALL?????index?????????????????
????range??range Index Scan????????????杩�???????????????????校???????between??<??>???????
????ref????????????瑁�???????????????????????小?????????梅???????????????????械?????
????eq_ref??????????瑁�?????????????????????????????????????????????????????
????const??system????MySQL??????????????????????????????????????些?????????缃�????????where?斜??校?MySQL????貌?????????????
????NULL??MySQL??????????蟹????????????????梅?????????
????possible_keys?????MySQL?????????????????????校?????婕�????????????????????????????????谐???????????????????
????key?????MySQL?????????????????????????????????????NULL????????????????????????????????????????key?斜???
????key_len??????????????????????????????屑????????????????????
????ref?????????????????????????????些?谢????????????????????????
????rows?????MySQL????????????????????????????????????????????????????????
????Extra??????????????????????????????????????????
????Using where?????MySQL????????娲�???????????????“?????”??Post-filter??????????未???????????Using where?????????????????MySQL????where?????????????
????Using temporary?????MySQL??????????????娲�?????????????????????????
????Using filesort??MySQL???????????????????????????“???????”??
????2.3 mysql??屑???????
????EXPLAIN????????????????????娲�?????????????????搴�??????????????
????EXPLAIN?????????Cache
????EXPLAIN???????MySQL????胁????????????????
????????????????????????????
????EXPALIN??????SELECT?????????????????写?SELECT?????屑??