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alter system flush oracle的缓存
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下面3條語(yǔ)句，旨在刷新oracle的緩存。這里總結(jié)一下。
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??? 1）alter system flush global context
??? 下圖說(shuō)明：
??? 對(duì)于多層架構(gòu)的，如上圖：應(yīng)用服務(wù)器和數(shù)據(jù)塊服務(wù)器通過(guò)連接池進(jìn)行通信，對(duì)于連接池的這些信息被保留在SGA中，這條語(yǔ)句便是把這些連接信息清空。
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??? 2）alter system flush shared_pool
??? 將使library cache和data dictionary cache以前保存的sql執(zhí)行計(jì)劃全部清空，但不會(huì)清空共享sql區(qū)或者共享pl/sql區(qū)里面緩存的最近被執(zhí)行的條目。刷新共享池可以幫助合并碎片（small chunks），釋放少數(shù)共享池資源，暫時(shí)解決shared_pool中的碎片問(wèn)題。但是，這種做法通常是不被推薦的。原因如下：
??????????????? ·Flush Shared Pool會(huì)導(dǎo)致當(dāng)前未使用的cursor被清除出共享池，如果這些SQL隨后需要執(zhí)行，那么數(shù)據(jù)庫(kù)將經(jīng)歷大量的硬解析，系統(tǒng)將會(huì)經(jīng)歷嚴(yán)重的CPU爭(zhēng)用，數(shù)據(jù)庫(kù)將會(huì)產(chǎn)生激烈的Latch競(jìng)爭(zhēng)。
??????????????? ·如果應(yīng)用沒(méi)有使用綁定變量，大量類似SQL不停執(zhí)行，那么Flush Shared Pool可能只能帶來(lái)短暫的改善，數(shù)據(jù)庫(kù)很快就會(huì)回到原來(lái)的狀態(tài)。
??????????????? ·如果Shared Pool很大，并且系統(tǒng)非常繁忙，刷新Shared Pool可能會(huì)導(dǎo)致系統(tǒng)掛起，對(duì)于類似系統(tǒng)盡量在系統(tǒng)空閑時(shí)進(jìn)行。
??? 下面測(cè)試一下，刷新對(duì)共享池碎片的影響：
[sql]?view plaincopyprint?
SQL>?select?count(*)?from?x$ksmsp;?? ?? ??COUNT(*)?? ?? ?????41637?? ?? SQL>?alter?system?flush?shared_pool;?? ?? 系統(tǒng)已更改。?? ?? SQL>?select?count(*)?from?x$ksmsp;?? ?? ??COUNT(*)?? ?? ??????9276??
????3）alter system flush buffer_cache
??? 為了最小化cache對(duì)測(cè)試實(shí)驗(yàn)的影響，需要手動(dòng)刷新buffer cache，以促使oracle重新執(zhí)行物理訪問(wèn)（統(tǒng)計(jì)信息里面的：physical reads）。
??? 測(cè)試環(huán)境
[sql]?view plaincopyprint?
SQL>?select?count(*)?from?tt;?? ?? ??COUNT(*)?? ?? ???1614112?? ?? SQL>?show?user;?? USER?為?"HR"?? SQL>?exec?dbms_stats.gather_table_stats('HR','TT');?? ?? PL/SQL?過(guò)程已成功完成。?? ?? SQL>?select?blocks,empty_blocks?from?dba_tables?where?table_name='TT'?and?owner='HR';?? ?? ????BLOCKS?EMPTY_BLOCKS?? ?? ?????22357????????????0?? 表TT共有22357個(gè)block??
??? 借助x$bh，觀察state=0的情況
[sql]?view plaincopyprint?
SQL>?select?count(*)?from?x$bh?where?state=0;?? ?? ??COUNT(*)?? ?? ?????????0?? ?? SQL>?alter?system?flush?buffer_cache;?? ?? 系統(tǒng)已更改。?? ?? SQL>?select?count(*)?from?x$bh?where?state=0;?? ?? ??COUNT(*)?? ?? ?????40440??
??? state=0表示buffer狀態(tài)是free，flush cache后，所有的buffer都被標(biāo)志為free
??? 觀察flush cache后，對(duì)查詢的影響：
[sql]?view plaincopyprint?
SQL>?set?autot?on?statistics?? SQL>?select?count(*)?from?tt;?? ?? ??COUNT(*)?? ?? ???1614112?? ?? ?? 統(tǒng)計(jì)信息?? ?? ??????????0??recursive?calls?? ??????????0??db?block?gets?? ??????22288??consistent?gets?? ??????22277??physical?reads?? ??????????0??redo?size?? ????????416??bytes?sent?via?SQL*Net?to?client?? ????????385??bytes?received?via?SQL*Net?from?client?? ??????????2??SQL*Net?roundtrips?to/from?client?? ??????????0??sorts?(memory)?? ??????????0??sorts?(disk)?? ??????????1??rows?processed?? ?? SQL>?/?? ?? ??COUNT(*)?? ?? ???1614112?? ?? ?? 統(tǒng)計(jì)信息?? ?? ??????????0??recursive?calls?? ??????????0??db?block?gets?? ??????22288??consistent?gets?? ??????????0??physical?reads?? ??????????0??redo?size?? ????????416??bytes?sent?via?SQL*Net?to?client?? ????????385??bytes?received?via?SQL*Net?from?client?? ??????????2??SQL*Net?roundtrips?to/from?client?? ??????????0??sorts?(memory)?? ??????????0??sorts?(disk)?? ??????????1??rows?processed?? ?? SQL>?alter?system?flush?buffer_cache;?? ?? 系統(tǒng)已更改。?? ?? SQL>?select?count(*)?from?tt;?? ?? ??COUNT(*)?? ?? ???1614112?? ?? ?? 統(tǒng)計(jì)信息?? ?? ??????????0??recursive?calls?? ??????????0??db?block?gets?? ??????22288??consistent?gets?? ??????22277??physical?reads?? ??????????0??redo?size?? ????????416??bytes?sent?via?SQL*Net?to?client?? ????????385??bytes?received?via?SQL*Net?from?client?? ??????????2??SQL*Net?roundtrips?to/from?client?? ??????????0??sorts?(memory)?? ??????????0??sorts?(disk)?? ??????????1??rows?processed ?
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