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Oracle Team

What’s the consequence of NOLOGGING loads?

By | Database management, Oracle | No Comments

By Franck Pachot . When you load data in direct-path and have the NOLOGGING attribute set, you minimize redo generation, but you take the risk, in case of media recovery, to loose the data in the blocks that you’ve loaded. So you probably run a backup as soon as the load is done. But what happens if you have a crash, with media failure, before the backup is finish? I encountered recently the situation but…

 
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Oracle Team

Oracle 12.1.0.2.1 Set to Join Conversion

By | Database management, Oracle | No Comments

By Franck Pachot . Recently, I described the Partial Join Evaluation transformation that appeared last year in Oracle 12c. I did it as an introduction for another transformation that appeared long time ago in 10.1.0.3 but was not used by default. And even in the latest Oracle 12c patchset 1 (aka 12.1.0.2.0) it is still not enabled. But it’s there and you can use it if you set optimizer_features_enabled to 12.1.0.2.1 (that’s not a typo!)….

 
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Oracle Team

Oracle 12.1.0.2: Wait event histograms in μs

By | Database Administration & Monitoring, Oracle | 2 Comments

By Franck Pachot . When an Oracle Database spends a high percentage of its DB time in User I/O, I usually check the wait event histograms in order to see if the storage system is working well. But today, with storage going to SSD, most I/O are less than 1 milliseconds and we have no details about those wait times. Here is what is exposed by V$WAIT_EVENT_HISTOGRAM:   select event,wait_time_milli,wait_count from v$event_histogram where event like…

 
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Oracle Team

PDB media failure may cause the whole CDB to crash

By | Database management, Oracle | 3 Comments

By Franck Pachot . Do you remember last year, when 12c arrived with multitenant, David Hueber warned us about the fact that a single PDB can, under certain conditions, generate a complete system downtime? We are beta testers and opened a SR for that. Now one year later the first patchset is out and obviously I checked if the issue was fixed. It’s a patchset afterall, which is expected to fix issues before than bringing…

 
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Oracle Team

Drilling down V$RECOVERY_AREA_USAGE

By | Database management, Oracle | 2 Comments

By Franck Pachot . In a previous post I used X$KCCAGF to get more information about reclaimable archived logs in FRA, because there is a bug in standby (not opened) databases where archivelog deletion policy is ignored. I explained that the view V$RECOVERY_AREA_USAGE has only aggregated information about space reclaimable without the details about which files are reclaimable or not. Here I’ll explain how I came to X$KCCAGF and I’ll give the query to get…

 
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Oracle Team

Oracle EM agent 12c thread leak on RAC

By | Database management, Oracle | 4 Comments

By Franck Pachot . In a previous post about nproc limit, I wrote that I had to investigate the nproc limit with the number of threads because my Oracle 12c EM agent was having thousands of threads. This post is a short feedback about this issue and the way I have found the root cause. It concerns the enterprise manager agent 12c on Grid Infrasctructure >= 11.2.0.2 NLWP The issue was: ps -o nlwp,pid,lwp,args -u…

 
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Oracle Team

Oracle Parallel Query: Did you use MapReduce for years without knowing it?

By | Database management, Oracle | No Comments

By Franck Pachot . I’ve read this morning that MapReduce is dead. The first time I heard about MapReduce was when a software architect proposed to stop writing SQL on Oracle Database and replace it with MapReduce processing. Because the project had to deal with a huge amount of data in a small time and they had enough budget to buy as many cores as they need, they wanted the scalability of parallel distributed processing….

 
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Oracle Team

Linux: how to monitor the nofile limit

By | Operation systems, Oracle | One Comment

By Franck Pachot . In a previous post I explained how to measure the number of processes that are generated when a fork() or clone() call checks the nproc limit. There is another limit in /etc/limits.conf – or in /etc/limits.d – that is displayed by ‘ulimit -n’. It’s the number of open files – ‘nofile’ – and here again we need to know what kind of files are counted. nofile ‘nofile’ is another limit that…

 
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Oracle Team

How to list all Oracle system schemas

By | Database management, Oracle | 4 Comments

By Franck Pachot . Do you want to know which users come with the Oracle Database and which are the ones you have created? This is that not easy, especially prior to Oracle 12c. You know SYS and SYSTEM, but there are many others – especially when you have installed all options. And it is important to know them all, e. g. when you import or apply a password policy, etc. Let’s see the options…

 
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Oracle Team

Oracle Exadata – poor optimization for FIRST_ROWS

By | Database Administration & Monitoring, Oracle | 3 Comments

By Franck Pachot . In a previous blog, I discussed the difference between rownum and row_number(), in particular their behaviour in implicitely adding a first_rows(n) to the optimizer. That reminded me that I forgot to blog about an issue I encountered and which concerns both approaches. It was on an Exadata: a nice full table scan with smartscan was taking a long time. And forcing to an index access – with a very bad index…

 
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