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oracle 10g的DBMS_XPLAN包中display_cursor函數(shù)不同于display函數(shù)，display_cursor用于顯示SQL語(yǔ)句的真實(shí)的執(zhí)行計(jì)劃，在大多數(shù)情況下，
顯示真實(shí)的執(zhí)行計(jì)劃有助于更好的分析SQL語(yǔ)句的全過(guò)程，尤其是運(yùn)行此SQL語(yǔ)句實(shí)時(shí)的I/O開(kāi)銷。通過(guò)對(duì)比預(yù)估的I/O與真實(shí)的I/O開(kāi)銷來(lái)判斷
SQL語(yǔ)句所存在問(wèn)題，如缺少統(tǒng)計(jì)信息，SQL語(yǔ)句執(zhí)行的次數(shù)，根據(jù)實(shí)際中間結(jié)果集的大小來(lái)選擇合適的連接方式等。本文僅僅講述
display_cursor函數(shù)的使用。
一、display_cursor函數(shù)用法
1、display_cursor函數(shù)語(yǔ)法
DBMS_XPLAN.DISPLAY_CURSOR(
sql_id IN VARCHAR2 DEFAULT NULL,
cursor_child_no IN NUMBER DEFAULT NULL,
format IN VARCHAR2 DEFAULT 'TYPICAL');
2、display_cursor函數(shù)參數(shù)描述
sql_id
指定位于庫(kù)緩存執(zhí)行計(jì)劃中SQL語(yǔ)句的父游標(biāo)。默認(rèn)值為null。當(dāng)使用默認(rèn)值時(shí)當(dāng)前會(huì)話的最后一條SQL語(yǔ)句的執(zhí)行計(jì)劃將被返回
可以通過(guò)查詢V$SQL 或V$SQLAREA的SQL_ID列來(lái)獲得SQL語(yǔ)句的SQL_ID。
cursor_child_no
指定父游標(biāo)下子游標(biāo)的序號(hào)。即指定被返回執(zhí)行計(jì)劃的SQL語(yǔ)句的子游標(biāo)。默認(rèn)值為0。如果為null，則sql_id所指父游標(biāo)下所有子游標(biāo)
的執(zhí)行計(jì)劃都將被返回。
format
控制SQL語(yǔ)句執(zhí)行計(jì)劃的輸出部分，即哪些可以顯示哪些不顯示。使用與display函數(shù)的format參數(shù)與修飾符在這里同樣適用。
除此之外當(dāng)在開(kāi)啟statistics_level=all時(shí)或使用gather_plan_statistics提示可以獲得執(zhí)行計(jì)劃中實(shí)時(shí)的統(tǒng)計(jì)信息
有關(guān)詳細(xì)的format格式描述請(qǐng)參考：dbms_xplan之display函數(shù)的使用 中format參數(shù)的描述
下面給出啟用統(tǒng)計(jì)信息時(shí)format新增的修飾符
iostats 控制I/O統(tǒng)計(jì)的顯示
last 默認(rèn)，顯示所有執(zhí)行計(jì)算過(guò)的統(tǒng)計(jì)。如果指定該值，則只顯示最后一次執(zhí)行的統(tǒng)計(jì)信息
memstats 控制pga相關(guān)統(tǒng)計(jì)的顯示
allstats 此為iostats memstats的快捷方式，即allstats包含了iostats和memstats
run_stats_last 等同于iostats last。只能用于oracle 10g R1
run_stats_tot 等同于iostats。只能用于oracle 10g R1
抓一個(gè)最近一小時(shí)最消耗IO的SQL：
SELECT sql_id, COUNT(*)
FROM gv$active_session_history ash, gv$event_name evt
WHERE ash.sample_time > SYSDATE - 1 / 24
AND ash.session_state = 'WAITING'
AND ash.event_id = evt.event_id
AND evt.wait_class = 'User I/O'
GROUP BY sql_id
ORDER BY COUNT(*) DESC;
執(zhí)行上面的SQL：
SQL> SELECT sql_id, COUNT(*)
FROM gv$active_session_history ash, gv$event_name evt
2 3 WHERE ash.sample_time > SYSDATE - 1 / 24
4 AND ash.session_state = 'WAITING'
5 AND ash.event_id = evt.event_id
6 AND evt.wait_class = 'User I/O'
7 GROUP BY sql_id
8 ORDER BY COUNT(*) DESC;
SQL_ID COUNT(*)
------------- ----------
g7fu6qba82m6b 668
63r47zyphdk06 526
9f5m4wd88nc1h 514
593p47drw5fhk 232
br91w16jzy4fu 120
4fvwyjpnh6tp7 78
gm0nrbfuj8kzr 70
2184k363hw4xd 68
gc4dajs7g5myy 46
8vrk9sfuwfdgq 42
ccpnb4dwdmq21 40
查看SQL的執(zhí)行計(jì)劃：
SELECT * FROM TABLE(dbms_xplan.display_cursor('g7fu6qba82m6b'));
在SQLPLUS中執(zhí)行：
SQL> set pagesize 2000
SQL> SELECT * FROM TABLE(dbms_xplan.display_cursor('g7fu6qba82m6b'));
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------
SQL_ID g7fu6qba82m6b, child number 0
-------------------------------------
UPDATE "CPDDS_PDATA"."CDM_LEDGER" SET "CSTM_NAME" = :a1,"CSTM_NO" =
:a2,"PAPER_TYPE" = :a3,"PAPER_NO" = :a4,"CURR_TYPE" = :a5,"SVT_NO" =
:a6,"BAL_DIR" = :a7,"BAL" = :a8,"AVAL_BAL" = :a9,"NORM_FRATIO" =
:a10,"PK_BAL" = :a11,"DR_ACCU" = :a12,"CR_ACCU" = :a13,"LAST_TRAN_DATE" =
:a14,"LAST_TRAN_TIME" = :a15,"PRT_LINE_NUM" = :a16,"NOREG_PK_REC_NUM" =
:a17,"PK_NO" = :a18,"PWD" = :a19,"FLAG" = :a20,"FRZ_FLAG" =
:a21,"CARD_HOLD_FLAG" = :a22,"PK_HOLD_FLAG" = :a23,"BGN_INT_DATE" =
:a24,"OPEN_DATE" = :a25,"ACC_HOLD_FLAG" = :a26,"CLS_DATE" =
:a27,"OPEN_TLR" = :a28,"CLS_TLR" = :a29,"CLS_INT" = :a30,"OPEN_INST" =
:a31,"ADD_NUM" = :a32,"DAC" = :a33,"FRZ_TIMES1" = :a34,"FRZ_TIMES2" =
:a35,"HOST_SEQNO" = :a36,"D_UPDATE_DATE" = :a37 WHERE "ACC" = :b0
Plan hash value: 319441092
-----------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------
| 0 | UPDATE STATEMENT | | | | 3 (100)| |
| 1 | UPDATE | CDM_LEDGER | | | | |
|* 2 | INDEX UNIQUE SCAN| I_CDM_LEDGER | 1 | 269 | 2 (0)| 00:00:01 |
-----------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("ACC"=:B0)
29 rows selected.
總結(jié)
1、與display函數(shù)不同，display_cursor顯示的為真實(shí)的執(zhí)行計(jì)劃
2、對(duì)于format參數(shù)，使用與display函數(shù)的各個(gè)值，同樣適用于display_cursor函數(shù)
3、當(dāng)statistics_level為all或使用gather_plan_statistics提示可以獲得執(zhí)行時(shí)的統(tǒng)計(jì)信息
4、根據(jù)真實(shí)與預(yù)估的統(tǒng)計(jì)信息可以初步判斷SQL效率低下的原因，如統(tǒng)計(jì)信息的準(zhǔn)確性、主要的開(kāi)銷位于那些步驟等
舉報(bào)