In the previous post https://blog.dbi-services.com/12c-multitenant-internals-pdb-replay-ddl-for-common-users I’ve done some DDL on a common user to show how this is replayed later for PDBs that were not opened at that time. But what happens when one of the DDL fails on one PDB?
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In multitenant, you can create common Users, Roles, and Profiles. You create them in CDB$ROOT, with the CONTAINER=ALL clause (which is optional because it is the only possible value when connected to CDB$ROOT) but they are visible to all containers. As the goal of multitenant is to avoid to duplicate common metadata to all containers, You may think that they are visible through those magic metadata links. But that’s actually wrong: they are simply replicated…
This month, I’ll talk – with lot of demos – about multitenant internals at DOAG conference. The multitenant dictionary architecture starts with a simple idea: system metadata and data are in CDB$ROOT and user metadata and data are in PDB. And it could have been that simple. When a session connected to a PDB needs to read some system information, the session context is switched to the CDB$ROOT container and reads from CDB$ROOT SYS tablespace…
The documentation says that for Reduction of duplication and Ease of database upgrade the Oracle-supplied objects such as data dictionary table definitions and PL/SQL packages are represented only in the root. Unfortunately, this is only partly true. System PL/SQL packages are only in root but system table definition are replicated into all PDBs.
I described in an earlier post on AWR views how the dictionary views were using metadata and object links to show information from other containers. But this mechanism cannot work for fixed views (aka V$) because they don’t have their definition in the dictionary.
On 31st of May in Düsseldorf, at DOAG Datenbank, I’ll talk about transportable tablespaces and pluggable databases. Both methods are transporting data physically, the difference is in the transport of the metadata, which can be more flexible when transported logically, as with TTS, but faster when transported physically with PDB. I have a lot of demos to show transportable tablespaces with RMAN, and the different cloning features available in 12cR2. If I have time I’ll…
Today at UKOUG TECH16 conference I’m presenting the internals of the new multitenant architecture: 12c Multitenant: Not a Revolution, Just an Evolution. My goal is to show how it works, that metadata links and object links are not blind magic. Here are the links to the blog posts I’ve published about multitenant internals.
The previous post partly answered to the original question (why an object link to V$SESSION is refreshed only every 30 seconds): recursive queries on shared=object views. Now let’s see what is different with fixed tables.
I had a comment on object links internals when creating a sharing=object view on GV$SESSION. Before posting about this specific case, I realized that I’ve never explained how a query on an object link is run on the CDB$ROOT container.