SQL join problem caused by Low cardinality with Histogram(柱状图产生的过低Card.导致表join性能问题)(二)

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接上一篇 http://www.anbob.com/archives/2323.html 下面我们从统计信息入手 sys@DEVDB>select count(distinct join_course_count),count(distinct is_course_qual),count(*) from app_zyyjhj.em_examinee; COUNT(DISTINCTJOIN_COURSE_COUNT) COUNT(DISTINCTIS_COURSE_QUAL) COUNT(*) ——————————– —————————– ——————– 6 4 35765 #使用tabstat2 脚本收信表信息，我已截断输出 sys@DEVDB>@tabstat2 em_examinee app_zyyjhj Table Number Empty Average Chain Average Global User Sample Date Name of Rows Blocks Blocks Space Count Row Len Stats Stats Size MM-DD-YYYY ————— ————– ——– ———— ——- ——– ——- —— —— … Read more

The filter pushed in subquery issue in oracle 11G (filter推进子查询)

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注意上面的4# 计划 的fileter 部分， 这就是问题的根源。了解SQL的执行步骤应该清楚在sql parse阶段CBO会帮我们做sql查询转换生成高效的执行计划, 在本案例我们想把TO_NUMBER(“S”.”SCORE”)<60 的条件放到子查询返回的结果集后再过滤, 结果CBO错误把此条件提前和该字段的其它条件一并执行..

What is GridFS?

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What is it ?

a way to store files in your database that doesn’s suck.
a way to ensure that metadata is always kept with a file.
a way to get all the scaliing benefits of mongodb for files.

How to install MongoDB 2.6 on linux

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官网地址:http://www.mongodb.org; 1. Download the binary files for the desired release of MongoDB. wget http://124.202.164.12/files/113100000380F0C3/fastdl.mongodb.org/linux/mongodb-linux-x86_64-2.6.0.tgz 2. Extract the files from the downloaded archive. tar zxvf mongodb-linux-x86_64-2.6.0.tgz 3.Add new user ,Copy the extracted archive to the target directory. useradd mongo mkdir -p /mongodb mkdir -p /data/db chown -R mongo.mongo /mongodb chown -R mongo.mongo /data/db cp -R -n … Read more

How to duplicate or multiplex control files on ASM ?(复制控制文件)

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ON single-instance 1, make sure asm diskgroup is avaliable Select name,total_mb,free_mb from v$asm_diskgroup where state=’CONNECTED’; 2, check current controlfile and using spfile show parameter control_files show parameter pfile 3, add new controlfile to spfile alter system set control_files='[original file]’,'[asm diskgroup name where new control file will stored]’ scope=spfile ; for example: alter system set control_files=’+OLD/ANBOB/controlfile/current.258.798205861′,’+NEW_DATA’ … Read more

RedHat(RHEL)6 support ASMLIB (RHEL6支持asmlib)

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With the annoucement of Oracle’s database support on RHEL6 and OEL6, they have decided to stop making ASMLIB for RHEL compatible kernels. ASMLIB is now only available if you are using Oracle Unbreakable Linux Kernel.But Recent events enabled Red Hat and Oracle to work togethe. Oracle ASMLib availability and support on Red Hat Enterprise Linux 6.