ORACLE SQL调优案例一则
收到监控告警日志文件（Alert）的作业发出的告警邮件，表空间TEMPSCM2不能扩展临时段，说明临时表空间已经被用完了，TEMPSCM2表空间不够用了
Dear All:
The Instance SCM2‘ alert log occured the ora errors ,please see the detail blow and take action for it. many thanks!
------------------------------------------- The errors is blow ------------------------------------------------------
193 | | ORA-1652: unable to extend temp segment by 128 in tablespace TEMPSCM2
198 | | ORA-1652: unable to extend temp segment by 128 in tablespace TEMPSCM2
200 | | ORA-1652: unable to extend temp segment by 128 in tablespace TEMPSCM2
205 | | ORA-1652: unable to extend temp segment by 128 in tablespace TEMPSCM2
--------------------------------------------end of errors-----------------------------------------------------------
Oracle Alert Services
同事在分析处理时，定位到临时表空间是被一个问题SQL语句给耗尽了。
SELECT B.TABLESPACE, B.SEGFILE#, B.SEGBLK#, B.BLOCKS, A.SID, A.SERIAL#, A.USERNAME, A.OSUSER, A.STATUSFROM v$session A, v$tempseg_usage B, v$sqlarea C
WHERE A.saddr = B.session_addrAND C.address= A.sql_addressAND C.hash_value = A.sql_hash_valueORDER BY B.tablespace, B.blocks;
WORKLOAD REPOSITORY SQL Report显示，单个该SQL的HASH GROUP BY操作就要耗用临时表空间229M，他给的建议是不扩展TEMPSCM2表空间，而是去优化这个SQL语句，因为大部分时候，该数据库的临时表空间使用率是非常低。我也同意他的分析结果。
从该SQL语句的执行计划，就能看出这个SQL语句有问题，例如SC_LOT、PO_HD全表扫描只是为了获取一小部分数据。
SELECT ‘CEG‘ AS FTY_CD,
2015 AS PD_Year, 2 AS PD_Month, a.po_no,
SUM(a.total_qty) AS Order_Qty,
SUM(c.total_qty) GO_QTY, b.buyer_po_del_date,
b.status,
c.sam_group_cd,
c.style_chn_desc,
Max(e.short_name) AS CUSTOMER
FROM sc_lot a, po_hd b,
sc_hd c,
gen_customer e,
(SELECT ct_no AS Job_order_no
FROM mars_upload_temp WHERE fty_cd = ‘CEG‘
AND pd_yr = 2015 AND pd_mth = 2 AND date_type = ‘20150529 10:00:23881698737881698737‘) d
WHERE Upper(a.po_no) = Upper(b.po_no)
AND b.sc_no = c.sc_no AND Upper(a.po_no) = Upper(d.job_order_no)
AND c.customer_cd = e.customer_cd(+) GROUP BY b.buyer_po_del_date,
b.status,
c.sam_group_cd,
c.style_chn_desc,
a.sc_no,
a.po_no
了解了该语句的业务逻辑并和开发人员沟通后，发现WHERE语句的条件Upper函数根本没有必要，取消Upper函数后PO_HD、GEN_CUSTOMER表走索引扫描了
SELECT ‘CEG‘ AS FTY_CD,
2015 AS PD_Year, 2 AS PD_Month, a.po_no,
SUM(a.total_qty) AS Order_Qty,
SUM(c.total_qty) GO_QTY, b.buyer_po_del_date,
b.status,
c.sam_group_cd,
c.style_chn_desc,
Max(e.short_name) AS CUSTOMER
FROM sc_lot a, po_hd b,
sc_hd c,
gen_customer e,
(SELECT ct_no AS Job_order_no
FROM mars_upload_temp WHERE fty_cd = ‘CEG‘
AND pd_yr = 2015 AND pd_mth = 2 AND date_type = ‘20150529 10:00:23881698737881698737‘) d
WHERE a.po_no= b.po_no AND b.sc_no = c.sc_no AND a.po_no= d.job_order_no AND c.customer_cd = e.customer_cd(+) GROUP BY b.buyer_po_del_date,
b.status,
c.sam_group_cd,
c.style_chn_desc,
a.sc_no,
a.po_no
但是SC_LOT表还是走全表扫描，经过分析发现SC_LOT表的PO_NO列的区分度非常大，应该可以通过建立索引优化。如下所示，建立索引后，SC_LOT不走全表扫描了。
执行计划的代价（Cost）也从7014降为了254. 优化的效果非常显著（Cardinality变得非常大，是因为表MARS_UPLOAD_TEMP数据在我测试阶段发生了变化）
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