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12c Release 1

Additional information on Oracle 12c big table caching

Teaching is on the things I like doing, and currently I am investigating the Oracle 12c features around caching data in the various memory areas. Since the In-Memory (cost) option has been discussed by other far more knowledgeable people I would like to share some findings about the big table caching here.

Some Background

In Oracle 12c you have two additional options to cache information: full database caching and big table caching. The first is great if you have a massively big machine with lots and lots of DRAM plus a clever OS that can deal with the inevitable ccNUMA setup you will have to tackle. And maybe don’t want to pay for the In-Memory option. This post is not about full database caching, but rather about the other possibility to cache blocks.

Exadata Fast Data File Creation

This post is the result of some testing I performed on Exadata data file creation. You may know that Exadata offloading incorporates SQL optimisations as well as some infrastructure work. For quite some time Exadata allowed the DBA to create data files a lot quicker than on traditional systems. This has been documented before, for example by @mpnsh here

Little things worth knowing: Is there a penalty in establishing a connection to Oracle using the MAA connection string? Part 2

In the very lengthy previous post about the MAA connect string I wanted to explain the use of the MAA connection string as promoted by Oracle. I deliberately kept the first part simple: both primary and standby cluster were up, and although the database was operating in the primary role on what I called standby cluster (again it’s probably not a good idea to include the intended role in the infrastructure names) there was no penalty establishing a connection.

Little things worth knowing: Is there a penalty in establishing a connection to Oracle using the MAA connection string?

Sorry for the long title!

I had a question during my session about “advanced RAC programming features” during the last Paris Oracle Meetup about the MAA connection string. I showed an example taken from the Appication Continuity White Paper ( Someone from the audience asked me if I had experienced any problems with it, such as very slow connection timeouts. I haven’t, but wanted to double-check anyway. This is a simplified test using a sqlplus connection since it is easier to time than a call to a connection pool creation. If you know of a way to reliably do so in Java/UCP let me know and I’ll test it.

Little things worth knowing: what does opatchauto actually do?

I recently applied system patch 20132450 to my installation on a 2 node RAC system on Oracle Linux 7.1. While ensuring that OPatch is the latest version available I came across an interesting command line option in opatchauto. It is called “-generateSteps”.

Little things worth knowing: direct path inserts and referential integrity

This is another post to remind myself that Oracle evolves, and what I thought I knew might no longer be relevant. So double-checking instead of assuming should become a habit!

Today’s example: direct path inserts. I seemed to remember from Oracle 9i that a direct path insert ignores referential integrity. This is still confirmed in the 9i Release 2 Concepts Guide, chapter 19 “Direct Path Insert”. Quoting from there:

During direct-path INSERT operations, Oracle appends the inserted data after existing data in the table. Data is written directly into datafiles, bypassing the buffer cache. Free space in the existing data is not reused, and referential integrity constraints are ignored

That sounds a bit harsh in today’s times so it’s worth a test. On Oracle I created a parent/child relationship, admittedly rather crude:

Little things worth knowing: exp/imp vs expdp and impdp for HCC in Exadata

Do you know the difference between exp/imp and expdp/impdp when it comes to importing HCC compressed data in Exadata?

If not, then follow me through two examples. This is on but applies to all database releases you can have on Exadata. The task at hand is to export a table (which happens to be non-partitioned and HCC compressed for query high) and import it into a different user’s schema. This is quite a common approach when migrating data from a non-Exadata system into an Exadata system. You could for example pre-create the DDL for the tables and implement HCC before even importing a single row. When importing the data, the partitions’ HCC attributes will be honoured and data will be inserted compressed. Or won’t it?

The table

What happens in ASM if usable_file_mb is negative and you lose a failgroup

Having read the excellent post “Demystifying ASM REQUIRED_MIRROR_FREE_MB and USABLE_FILE_MB” again by Harald von Breederode I wanted to see what happens if you create a setup where your usable_file_mb is negative and you actually have to rebalance after a fatal failgroup error. I am using on Oracle Linux 6.6/UEK3 in a KVM in case anyone is interested. I/O times aren’t stellar on that environment. It’s Oracle Restart, not clustered ASM.

Note: this post is only applicable if you are using ASM for data protection, e.g. normal or high redundancy. External redundancy is a different thing: if you lose a failgroup in a disk group with external redundancy defined then you are toast. The disk group will dismount on all ASM instances. No disk group = nothing to write to = crash of dependent databases.


How to resolve the text behind v$views?

This is a common problem I have and I never write it down (except now). For example, today I wanted to know what the valid parameters for _serial_direct_read were:

SQL> select * from v$parameter_valid_values where name ='_serial_direct_read';

no rows selected

OK so if Oracle doesn’t tell me then maybe I can work it out? Getting the view_text has worked in the past:

Adaptive plans and v$sql_plan and related views

Adaptive plans are one of the coolest new optimiser features in Oracle 12c. If you haven’t seen or heard about them in detail I recommend the following resources: