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一个通过添加本地分区索引提高SQL性能的案例
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今天接到同事求助，說有一個select query，在Oracle上要跑一分多鐘，他希望能在5s內出結果，該sql如下：
[sql]? view plain copy print ?
Select??/*+?parallel(src,?8)?*/?distinct?? ??src.systemname?as?systemname?? ??,??src.databasename?as?databasename?? ??,??src.tablename?as?tablename?? ??,??src.username?as?username?? from??<strong>meta_dbql_table_usage_exp_hst</strong>?src?? ?inner?<strong>join?DR_QRY_LOG_EXP_HST</strong>?rl?on?? ??<strong>src.acctstringdate?=?rl.acctstringdate?? ??and?src.queryid?=?rl.queryid</strong>?? ??And?Src.Systemname?=?Rl.Systemname?? ??and?src.acctstringdate?>?sysdate?-?30?? ??And?Rl.Acctstringdate?>?Sysdate?-?30?? ?inner?join??<strong>meta_dr_qry_log_tgt_all_hst?</strong>tgt?on?? ??upper(tgt.systemname)?=?upper('MOZART')?? ??And?Upper(tgt.Databasename)?=?Upper('GDW_TABLES')?? ??And?Upper(tgt.Tablename)?=?Upper('SSA_SLNG_LSTG_MTRC_SD')?? ??<strong>AND?src.acctstringdate?=?tgt.acctstringdate?? ??and?rl.statement_id?=?tgt.statement_id</strong>?? ??and?rl.systemname?=?tgt.systemname?? ??And?Tgt.Acctstringdate?>?Sysdate?-?30?? ??And?Not(?? ????Upper(Tgt.Systemname)=Upper(src.systemname)?? ????And?? ????Upper(Tgt.Databasename)?=?Upper(Src.Databasename)?? ????And?? ????Upper(Tgt.Tablename)?=?Upper(Src.Tablename)?? ????)?? ??And???tgt.Systemname?is?not?null?? ??And???tgt.Databasename?Is?Not?Null?? ??And???tgt.tablename?is?not?null?? ;??
SQL的簡單分析
總得來看，這個SQL就是三個表(meta_dbql_table_usage_exp_hst，DR_QRY_LOG_EXP_HST，meta_dr_qry_log_tgt_all_hst）的INNER JOIN，這三個表數據量都在百萬級別，且都是分區表（以acctstringdate為分區鍵），執行計劃如下：
[sql]? view plain copy print ?
?? |?Id??|?Operation??????????????????????????????|?Name??????????????????????????|?Rows??|?Bytes?|?Cost??|?Pstart|?Pstop?|?? ?? |???0?|?SELECT?STATEMENT???????????????????????|???????????????????????????????|?????1?|???159?|??8654?|???????|???????|?? |???1?|??PX?COORDINATOR????????????????????????|???????????????????????????????|???????|???????|???????|???????|???????|?? |???2?|???PX?SEND?QC?(RANDOM)??????????????????|?:TQ10002??????????????????????|?????1?|???159?|??8654?|???????|???????|?? |???3?|????SORT?UNIQUE?????????????????????????|???????????????????????????????|?????1?|???159?|??8654?|???????|???????|?? |???4?|?????PX?RECEIVE?????????????????????????|???????????????????????????????|?????1?|????36?|?????3?|???????|???????|?? |???5?|??????PX?SEND?HASH??????????????????????|?:TQ10001??????????????????????|?????1?|????36?|?????3?|???????|???????|?? |*??6?|???????TABLE?ACCESS?BY?LOCAL?INDEX?ROWID|?DR_QRY_LOG_EXP_HST????????????|?????1?|????36?|?????3?|???????|???????|?? |???7?|????????NESTED?LOOPS????????????????????|???????????????????????????????|?????1?|???159?|??8633?|???????|???????|?? |???8?|?????????NESTED?LOOPS???????????????????|???????????????????????????????|??8959?|??1076K|??4900?|???????|???????|?? |???9?|??????????BUFFER?SORT???????????????????|???????????????????????????????|???????|???????|???????|???????|???????|?? |??10?|???????????PX?RECEIVE???????????????????|???????????????????????????????|???????|???????|???????|???????|???????|?? |??11?|????????????PX?SEND?BROADCAST???????????|?:TQ10000??????????????????????|???????|???????|???????|???????|???????|?? |??12?|?????????????PARTITION?RANGE?ITERATOR???|???????????????????????????????|?????1?|????56?|??4746?|???KEY?|????14?|?? |*?13?|??????????????TABLE?ACCESS?FULL?????????|?META_DR_QRY_LOG_TGT_ALL_HST???|?????1?|????56?|??4746?|???KEY?|????14?|?? |??14?|??????????PX?BLOCK?ITERATOR?????????????|???????????????????????????????|??8959?|???586K|???154?|???KEY?|???KEY?|?? |*?15?|???????????TABLE?ACCESS?FULL????????????|?META_DBQL_TABLE_USAGE_EXP_HST?|??8959?|???586K|???154?|???KEY?|???KEY?|?? |??16?|?????????PARTITION?RANGE?ITERATOR???????|???????????????????????????????|?????1?|???????|?????2?|???KEY?|???KEY?|?? |*?17?|??????????INDEX?RANGE?SCAN??????????????|?DR_QRY_LOG_EXP_HST_IDX????????|?????1?|???????|?????2?|???KEY?|???KEY?|?? ?? Predicate?Information?(identified?by?operation?id):?? ?? ?? ???6?-?filter("RL"."STATEMENT_ID"="TGT"."STATEMENT_ID"?AND?"RL"."SYSTEMNAME"="TGT"."SYSTEMNAME"?AND?"SRC"."SYSTEMNAME"="RL"."SYSTEMNAME")?? ??13?-?filter(UPPER("TGT"."SYSTEMNAME")='MOZART'?AND?UPPER("TGT"."DATABASENAME")='GDW_TABLES'?AND?? ??????????????UPPER("TGT"."TABLENAME")='SSA_SLNG_LSTG_MTRC_SD'?AND?"TGT"."ACCTSTRINGDATE">SYSDATE@!-30?AND?"TGT"."SYSTEMNAME"?IS?NOT?NULL?? ??????????????"TGT"."DATABASENAME"?IS?NOT?NULL?AND?"TGT"."TABLENAME"?IS?NOT?NULL)?? ??15?-?filter("SRC"."ACCTSTRINGDATE"="TGT"."ACCTSTRINGDATE"?AND?(UPPER("TGT"."SYSTEMNAME")<>UPPER("SRC"."SYSTEMNAME")?OR?? ??????????????UPPER("TGT"."DATABASENAME")<>UPPER("SRC"."DATABASENAME")?OR?UPPER("TGT"."TABLENAME")<>UPPER("SRC"."TABLENAME"))?AND?? ??????????????"SRC"."ACCTSTRINGDATE">SYSDATE@!-30)?? ??17?-?access("SRC"."QUERYID"="RL"."QUERYID"?AND?"SRC"."ACCTSTRINGDATE"="RL"."ACCTSTRINGDATE")?? ???????filter("RL"."ACCTSTRINGDATE">SYSDATE@!-30)??
定位問題
從上面執行計劃中的表連接方式可以知道，這三個表之間進行了兩次NESTED LOOP，問題出現在最里層的NESTED LOOP（對兩個表都做了TABLE FULL SCAN），因為表都是百萬級別的（即時過濾后的數據量也不小），性能問題就出現在內表（即被驅動表）META_DBQL_TABLE_USAGE_EXP_HST做了太多次的全表掃描。如果能把全表掃描轉換成索引，則性能可以大幅度提高。
下面是NESTED LOOP的介紹：
嵌套連接把要處理的數據集分為外部循環（驅動數據源）和內部循環（被驅動數據源），外部循環只執行一次，內部循環執行的次數等于外部循環執行返回的數據個數。
這種連接的好處是內存使用非常少。
如果驅動數據源有限，且被驅動表在連接列上有相應的索引，則這種連接方式才是高效的。
下面是這三個表上索引的情況：
[sql]? view plain copy print ?
SQL>?select?index_name,?table_name?from?user_indexes?where?table_name?in?('DR_QRY_LOG_EXP_HST',upper('meta_dbql_table_usage_exp_hst'),?upper('meta_dr_qry_log_tgt_all_hs?? ?? INDEX_NAME???????????????????????????????????????????????????TABLE_NAME?? ?? META_DR_QRY_LOG_TGT_ALL_IDX??????????????????????????????????META_DR_QRY_LOG_TGT_ALL_HST?? META_DBQL_TUSAGE_EHST_IDX????????????????????????????????????META_DBQL_TABLE_USAGE_EXP_HST?? DR_QRY_LOG_EXP_HST_IDX???????????????????????????????????????DR_QRY_LOG_EXP_HST?? ?? CREATE?INDEX?"GV"."META_DR_QRY_LOG_TGT_ALL_IDX"?ON?"GV"."META_DR_QRY_LOG_TGT_ALL_HST"?("STATEMENT_ID",?"ACCTSTRINGDATE")?? CREATE?INDEX?"GV"."META_DBQL_TUSAGE_EHST_IDX"?ON?"GV"."META_DBQL_TABLE_USAGE_EXP_HST"?("QUERYID",?"ACCTSTRINGDATE")?? CREATE?INDEX?"GV"."DR_QRY_LOG_EXP_HST_IDX"?ON?"GV"."DR_QRY_LOG_EXP_HST"?("QUERYID",?"ACCTSTRINGDATE")??
這三個索引都是本地分區索引（都包含分區鍵acctstringdate），很顯然，DR_QRY_LOG_EXP_HST表少了個索引，因為它與表meta_dr_qry_log_tgt_all_hst 在statement_id上做join，因此應該在它的statement_id上也創建本地分區索引如下：
[sql]? view plain copy print ?
create?index?DR_QRY_LOG_EXP_HST_IDX2?on?gv.DR_QRY_LOG_EXP_HST?(statement_id,ACCTSTRINGDATE)?local;?? 性能對比
新的執行計劃如下：
[sql]? view plain copy print ?
?? |?Id??|?Operation??????????????????????????????|?Name??????????????????????????|?Rows??|?Bytes?|?Cost??|?Pstart|?Pstop?|?? ?? |???0?|?SELECT?STATEMENT???????????????????????|???????????????????????????????|?????1?|???159?|??4838?|???????|???????|?? |???1?|??SORT?UNIQUE???????????????????????????|???????????????????????????????|?????1?|???159?|??4838?|???????|???????|?? |*??2?|???TABLE?ACCESS?BY?LOCAL?INDEX?ROWID????|?META_DBQL_TABLE_USAGE_EXP_HST?|?????1?|????67?|?????3?|???????|???????|?? |???3?|????NESTED?LOOPS????????????????????????|???????????????????????????????|?????1?|???159?|??4816?|???????|???????|?? |???4?|?????NESTED?LOOPS???????????????????????|???????????????????????????????|????18?|??1656?|??4762?|???????|???????|?? |???5?|??????PARTITION?RANGE?ITERATOR??????????|???????????????????????????????|?????1?|????56?|??4746?|???KEY?|????14?|?? |*??6?|???????TABLE?ACCESS?FULL????????????????|?META_DR_QRY_LOG_TGT_ALL_HST???|?????1?|????56?|??4746?|???KEY?|????14?|?? |???7?|??????PARTITION?RANGE?ITERATOR??????????|???????????????????????????????|????18?|???648?|????16?|???KEY?|????14?|?? |*??8?|???????TABLE?ACCESS?BY?LOCAL?INDEX?ROWID|?DR_QRY_LOG_EXP_HST????????????|????18?|???648?|????16?|???KEY?|????14?|?? |*??9?|????????<strong>INDEX?RANGE?SCAN????????????????|?DR_QRY_LOG_EXP_HST_IDX2</strong>???????|????31?|???????|????15?|???KEY?|????14?|?? |??10?|?????PARTITION?RANGE?ITERATOR???????????|???????????????????????????????|?????1?|???????|?????2?|???KEY?|???KEY?|?? |*?11?|??????INDEX?RANGE?SCAN??????????????????|?META_DBQL_TUSAGE_EHST_IDX?????|?????1?|???????|?????2?|???KEY?|???KEY?|?? ?? ?? Predicate?Information?(identified?by?operation?id):?? ?? ?? ???2?-?filter((UPPER("TGT"."SYSTEMNAME")<>UPPER("SRC"."SYSTEMNAME")?OR?? ??????????????UPPER("TGT"."DATABASENAME")<>UPPER("SRC"."DATABASENAME")?OR?UPPER("TGT"."TABLENAME")<>UPPER("SRC"."TABLENAME"))?? ??????????????AND?"SRC"."SYSTEMNAME"="RL"."SYSTEMNAME")?? ???6?-?filter(UPPER("TGT"."SYSTEMNAME")='MOZART'?AND?UPPER("TGT"."DATABASENAME")='GDW_TABLES'?AND?? ??????????????UPPER("TGT"."TABLENAME")='SSA_SLNG_LSTG_MTRC_SD'?AND?"TGT"."ACCTSTRINGDATE">SYSDATE@!-30?AND?"TGT"."SYSTEMNAME"?? ??????????????IS?NOT?NULL?AND?"TGT"."DATABASENAME"?IS?NOT?NULL?AND?"TGT"."TABLENAME"?IS?NOT?NULL)?? ???8?-?filter("RL"."SYSTEMNAME"="TGT"."SYSTEMNAME")?? ???9?-?access("RL"."STATEMENT_ID"="TGT"."STATEMENT_ID"?AND?"RL"."ACCTSTRINGDATE">SYSDATE@!-30?AND?? ??????????????"RL"."ACCTSTRINGDATE"?IS?NOT?NULL)?? ??11?-?access("SRC"."QUERYID"="RL"."QUERYID"?AND?"SRC"."ACCTSTRINGDATE"="RL"."ACCTSTRINGDATE")?? ???????filter("SRC"."ACCTSTRINGDATE"="TGT"."ACCTSTRINGDATE"?AND?"SRC"."ACCTSTRINGDATE">SYSDATE@!-30)?? 從新的的執行計劃可以看出，它的第一個NESTED LOOP果然用了最新創建的索引。
下面是執行時間：
[plain]? view plain copy print ?
已用時間:??00:?00:?02.16??
兩秒種搞定，遠遠超出他期望的5s ：）
方法總結
NESTED LOOP高效的條件：
驅動數據源有限，且被驅動表在連接列上有相應的索引。
總結
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