再一次強調，ORACLE外鍵必須加索引

外鍵加索引是常識，必須牢記。本來不想寫這種簡單案例，但是連續遇到好幾起外鍵不加索引導致效能問題，所以還是寫一下。

一個兄弟問我 delete from Sa_Sales_Comm_Detail s where s.sales_commission_id=24240;  ---刪除105條資料非常慢，要跑幾十秒到上百秒

這個表總資料才35萬行，sales_commission_id 列有索引，執行計畫也確實是走了索引。 走索引返回105 條資料，不可能跑幾十秒跑上百秒的。

之後我問他 select * from Sa_Sales_Comm_Detail s where s.sales_commission_id=24240;   這個跑得慢嗎，他回答 0.06秒。

select * from Sa_Sales_Comm_Detail s where s.sales_commission_id=24240;  ---0.06秒

delete from Sa_Sales_Comm_Detail s where s.sales_commission_id=24240;   ---幾十秒幾百秒

遇到這種，直接做10046 trace，部分的trace檔案如下

=====================PARSING IN CURSOR #4 len=111 dep=1 uid=0 oct=3 lid=0 tim=1374414810328412 hv=4234506700 ad='99cc8678' select /*+ all_rows */ count(1) from "CMM"."SA_SALES_PER_SPLIT_DETAIL" where "SALES_COMMISSION_DETAIL_ID" = :1END OF STMTPARSE #4:c=4000,e=4619,p=0,cr=0,cu=2,mis=1,r=0,dep=1,og=1,tim=1374414810328406EXEC #4:c=999,e=1841,p=0,cr=0,cu=0,mis=1,r=0,dep=1,og=1,tim=1374414810330416FETCH #4:c=226965,e=221384,p=0,cr=6867,cu=0,mis=0,r=1,dep=1,og=1,tim=1374414810551844=====================PARSING IN CURSOR #2 len=106 dep=1 uid=0 oct=3 lid=0 tim=1374414810557316 hv=1936840180 ad='8ae35660' select /*+ all_rows */ count(1) from "CMM"."SA_SALES_COMM_REPROT" where "SALES_COMMISSION_DETAIL_ID" = :1END OF STMTPARSE #2:c=5000,e=5152,p=0,cr=0,cu=2,mis=1,r=0,dep=1,og=1,tim=1374414810557310EXEC #2:c=3000,e=2081,p=0,cr=0,cu=0,mis=1,r=0,dep=1,og=1,tim=1374414810559571WAIT #2: nam='db file scattered read' ela= 384 file#=5 block#=83604 blocks=5 obj#=104150 tim=1374414810560401WAIT #2: nam='db file scattered read' ela= 154 file#=5 block#=87785 blocks=8 obj#=104150 tim=1374414810561125WAIT #2: nam='db file scattered read' ela= 141 file#=5 block#=87802 blocks=7 obj#=104150 tim=1374414810561384WAIT #2: nam='db file scattered read' ela= 204 file#=5 block#=94633 blocks=8 obj#=104150 tim=1374414810561704WAIT #2: nam='db file scattered read' ela= 160 file#=5 block#=94642 blocks=7 obj#=104150 tim=1374414810562029WAIT #2: nam='db file scattered read' ela= 167 file#=5 block#=94649 blocks=8 obj#=104150 tim=1374414810562309WAIT #2: nam='db file scattered read' ela= 184 file#=5 block#=94674 blocks=7 obj#=104150 tim=1374414810562602WAIT #2: nam='db file scattered read' ela= 172 file#=5 block#=94705 blocks=8 obj#=104150 tim=1374414810562882WAIT #2: nam='db file scattered read' ela= 135 file#=5 block#=94714 blocks=7 obj#=104150 tim=1374414810563132WAIT #2: nam='db file scattered read' ela= 162 file#=5 block#=97529 blocks=8 obj#=104150 tim=1374414810563412WAIT #2: nam='db file scattered read' ela= 234 file#=5 block#=98314 blocks=7 obj#=104150 tim=1374414810563758WAIT #2: nam='db file scattered read' ela= 154 file#=5 block#=99513 blocks=8 obj#=104150 tim=1374414810564008WAIT #2: nam='db file scattered read' ela= 143 file#=5 block#=101666 blocks=7 obj#=104150 tim=1374414810564260WAIT #2: nam='db file scattered read' ela= 166 file#=5 block#=101681 blocks=8 obj#=104150 tim=1374414810564533WAIT #2: nam='db file scattered read' ela= 157 file#=5 block#=101690 blocks=7 obj#=104150 tim=1374414810564797WAIT #2: nam='db file scattered read' ela= 128 file#=5 block#=101697 blocks=8 obj#=104150 tim=1374414810565025WAIT #2: nam='db file scattered read' ela= 335 file#=5 block#=102027 blocks=16 obj#=104150 tim=1374414810565576WAIT #2: nam='db file scattered read' ela= 355 file#=5 block#=102043 blocks=16 obj#=104150 tim=1374414810566148WAIT #2: nam='db file scattered read' ela= 302 file#=5 block#=102059 blocks=16 obj#=104150 tim=1374414810566690WAIT #2: nam='db file scattered read' ela= 323 file#=5 block#=102075 blocks=16 obj#=104150 tim=1374414810567221WAIT #2: nam='db file scattered read' ela= 310 file#=5 block#=102091 blocks=16 obj#=104150 tim=1374414810567720WAIT #2: nam='db file scattered read' ela= 270 file#=5 block#=102107 blocks=16 obj#=104150 tim=1374414810568243WAIT #2: nam='db file scattered read' ela= 378 file#=5 block#=102123 blocks=16 obj#=104150 tim=1374414810568814WAIT #2: nam='db file scattered read' ela= 253 file#=5 block#=102139 blocks=14 obj#=104150 tim=1374414810569252WAIT #2: nam='db file scattered read' ela= 527 file#=5 block#=108043 blocks=16 obj#=104150 tim=1374414810570016WAIT #2: nam='db file scattered read' ela= 309 file#=5 block#=108059 blocks=16 obj#=104150 tim=1374414810570543WAIT #2: nam='db file scattered read' ela= 281 file#=5 block#=108075 blocks=16 obj#=104150 tim=1374414810571075WAIT #2: nam='db file scattered read' ela= 356 file#=5 block#=108091 blocks=16 obj#=104150 tim=1374414810571658WAIT #2: nam='db file scattered read' ela= 273 file#=5 block#=108107 blocks=16 obj#=104150 tim=1374414810572138WAIT #2: nam='db file scattered read' ela= 381 file#=5 block#=108123 blocks=16 obj#=104150 tim=1374414810572715WAIT #2: nam='db file scattered read' ela= 318 file#=5 block#=108139 blocks=16 obj#=104150 tim=1374414810573241WAIT #2: nam='db file scattered read' ela= 302 file#=5 block#=108155 blocks=14 obj#=104150 tim=1374414810573745WAIT #2: nam='db file scattered read' ela= 280 file#=5 block#=109195 blocks=16 obj#=104150 tim=1374414810574226WAIT #2: nam='db file scattered read' ela= 362 file#=5 block#=109211 blocks=16 obj#=104150 tim=1374414810574795WAIT #2: nam='db file scattered read' ela= 333 file#=5 block#=109227 blocks=16 obj#=104150 tim=1374414810575357WAIT #2: nam='db file scattered read' ela= 331 file#=5 block#=109243 blocks=16 obj#=104150 tim=1374414810575904WAIT #2: nam='db file scattered read' ela= 377 file#=5 block#=109259 blocks=16 obj#=104150 tim=1374414810576483WAIT #2: nam='db file scattered read' ela= 349 file#=5 block#=109275 blocks=16 obj#=104150 tim=1374414810577059WAIT #2: nam='db file scattered read' ela= 344 file#=5 block#=109291 blocks=16 obj#=104150 tim=1374414810577601WAIT #2: nam='db file scattered read' ela= 320 file#=5 block#=109307 blocks=14 obj#=104150 tim=1374414810578133WAIT #2: nam='db file scattered read' ela= 385 file#=7 block#=2699 blocks=16 obj#=104150 tim=1374414810578830

從trace檔案裡面看到，執行delete的時候，隱含的調用了

 select /*+ all_rows */ count(1) from "CMM"."SA_SALES_COMM_REPROT" where "SALES_COMMISSION_DETAIL_ID" = :1

這個sql 導致了 db file scattered read(多塊讀) ，也就是說 上面的sql 沒走索引，走了全表掃描 ，於是讓他在 SALES_COMMISSION_DETAIL_ID 列上面建立索引

建立索引之後，delete只需要0.幾秒即可。

反正記住，外鍵上面要建立索引。