Why does the upgrade of PDB$SEED always take longer?

Upgrading the PDB$SEED takes always longer than any other container.
What is the reason for an upgrade of the PDB$SEED taking so much longer in a real world environment?

How does an upgrade with Oracle Multitenant work?

I have written several blog posts in the past about the different techniques to upgrade an Oracle Multitenant environment:

1. Upgrade Everything At Once
2. Upgrade via Unplug/Plug/Upgrade

The interesting case happens in the “Everything At Once“. The PDB$SEED takes significantly longer to upgrade than any other container.

Upgrading the PDB$SEED takes always longer?

Quick recap: The PDB$SEED is the (I call it) blueprint PDB which gets used to provision a new PDB quickly as a clone of the PDB$SEED with some additions. It is (for users) always READ ONLY and UNTOUCHABLE. This means, as a user you are not allowed to touch or change or modify or manipulate it. If you still do so, you’ll hit “unsupported” territory.

Here you’ll find a complete example for an Everything At Once upgrade:

As the PDB$SEED is read only and untouchable, a design decision has been taken to RECOMPILE all invalid objects within the PDB$SEED container while the PDB$SEED is in UPGRADE mode. All other containers get recompiled AFTER the upgrade itself has been finished.

This is the reason why the upgrade of the PDB$SEED takes longer than expected.

Example

When you look into the log files post upgrade you will spot the reason for the longer duration of the PDB$SEED upgrade (marked in RED below):

Oracle Database 12.2 Post-Upgrade Status Tool           05-09-2017 14:46:01
                             [PDB1]

Component                               Current         Version  Elapsed Time
Name                                    Status          Number   HH:MM:SS

Oracle Server                          UPGRADED      12.2.0.1.0  00:32:28
Oracle Real Application Clusters       UPGRADED      12.2.0.1.0  00:00:00
Oracle XML Database                    UPGRADED      12.2.0.1.0  00:02:24
Final Actions                                                    00:06:59
Post Upgrade                                                     00:03:07

Total Upgrade Time: 00:45:31 [PDB1]


Oracle Database 12.2 Post-Upgrade Status Tool           05-09-2017 15:03:52
                             [PDB$SEED]

Component                               Current         Version  Elapsed Time
Name                                    Status          Number   HH:MM:SS

Oracle Server                             VALID      12.2.0.1.0  00:32:24
Oracle Real Application Clusters     OPTION OFF      12.2.0.1.0  00:00:00
Oracle XML Database                       VALID      12.2.0.1.0  00:02:25
Final Actions                                                    00:06:44
Post Upgrade                                                     00:03:16
Post Compile                                                     00:18:02

Total Upgrade Time: 01:03:24 [PDB$SEED]

The “Post Compile” phase does not exist for any other container as part of the upgrade.

–Mike