Underscore

Upgrade to 26ai may hang due to block checking events

This issue is caused by an internal change, and we hit this now with two very large customer upgrades recently. Therefore, I would like to point to a fix, a workaround and the following RU in case you see your Upgrade to 26ai may hang due to block checking events.

Upgrade to 26ai may hang due to block checking events

Photo by Nareeta Martin on Unsplash

What would you see?

This issue can happen with cloning, PDB refreshable clones or just a copy or restore of your database for the subsequent upgrade to Oracle AI Database 26ai. It is not dependent on AutoUpgrade but since AutoUpgrade is the only …

Datapatch return codes and helpful queries

Posted on February 17, 2026 by Mike.Dietrich datapatch 4

The other week I was visiting a customer in Germany who had specific questions for datapatch. And luckily, Alejandro Diaz who is the Product Manager for Data Pump in our team has a history in datapatch. He helped me with Datapatch return codes and helpful queries.

Photo by PuroClean of Fort Worth on Unsplash

Datapatch return codes

Yes, datapatch generates various return codes you can use to verify the outcome in case you program or maintain an automation.

0 for success
1 for prereq failure
2 for non-ignorable error during patch installation
255 runtime error

I may …

New interesting underscores for more than 255 _FIX_CONTROLs

Posted on April 22, 2025 by Mike.Dietrich Differences Optimizer Parameters 6

I keep saying this – we have the best and most skilled customers. Christian Klose from my Franconian home region contacted me via LinkedIn and asked for some assistance with an SR. I confess, I replied way too late (information overflow). But the finding Christian had is great to know. There are new interesting underscores for more than 255 _FIX_CONTROLs.

New interesting underscores for more than 255 _FIX_CONTROLs

Photo by Casey Horner on Unsplash

Why don’t you see them in ORAdiff?

As usual, I went to ORAdiff and tried to find out anything about these new _fix_control parameters. But I couldn’t find any indication for what Christian …

Queries on dictionary tables are slow in Oracle Multitenant?

Posted on February 10, 2025 by Mike.Dietrich Flaws and Pitfalls Single-/Multitenant 12

I love to learn something new every day. And on Friday afternoon last week I learned something new again. It started (as often) with a very simple question from a colleague working from the Oracle Malaga office in Spain. The customer has an issues with some queries being slower than before once they moved from on-prem to the cloud into a PDB. But are queries on dictionary tables are slow in Oracle Multitenant generally?

Queries on dictionary tables are slow in Oracle Multitenant?

Photo by Pascal van de Vendel on Unsplash

What was the case?

The customer ran some queries against dictionary views in the PDB in Oracle Database …

Oracle 12.2 and higher: Set _cursor_obsolete_threshold to old default

Posted on September 11, 2018 by Mike.Dietrich Parameters 18

Sometimes development groups change default parameter settings because it fits well for a specific feature. This has happened with _cursor_obsolete_threshold in Oracle Database 12.2. But there’s now an official recommendation for Oracle 12.2 and higher: Set _cursor_obsolete_threshold to old default.

What is _cursor_obsolete_threshold?

First of all, this is obviously an underscore parameter. And usually the policy is: Don’t change underscore parameters unless either you clearly understand what you are changing. Or unless Oracle Support (or another Oracle authority) advises you to do.

Oracle 12.2 and higher: Set _cursor_obsolete_threshold to old default

Photo by Jakub Gorajek on Unsplash

We introduced this parameter in Oracle 11.2.0.3 for an issue of growing …

Drop a tablespace in a PDB with a Guaranteed Restore Point being active

Posted on June 5, 2018 by Mike.Dietrich Parameters Single-/Multitenant 5

Drop a tablespace in a PDB with a Guaranteed Restore Point being active This was an interesting question I received from Alain Fuhrer a couple of weeks ago: “Why can’t I drop a tablespace in a PDB whit a Guaranteed Restore Point being active?”. It’s important to note that the GRP is not active in the same but in another PDB.

An SR resolved the issue. But in case you face the same situation, here’s the solution.

Drop a tablespace in a PDB with a Guaranteed Restore Point being active

Simple test scenario. These are my PDBs:

show pdbs

    CON_ID CON_NAME			  OPEN MODE  RESTRICTED
---------- ------------------------------ ---------- ----------
	 2 PDB$SEED			  READ ONLY

Remove Underscore Parameters not only before you upgrade

Posted on May 22, 2018 by Mike.Dietrich Parameters 7

Underscore parameters are a wonderful thing in Oracle. They are treated like a secret. But you can view them of course. Everybody can. Make a guess how many underscore parameters exist in Oracle 12.2.0.1.0. You’ll find the answer at the end of this blog post. But why should you remove underscore parameters not only before you upgrade?

I’d like to explain below why underscores, especially too many of them, may cause a lot of trouble. Therefore, housekeeping is very important.

Remove Underscore Parameters not only before you upgrade

This is from a recent upgrade from Oracle 12.1.0.2 to Oracle 12.2.0.1 …

Why EXCLUDE_SEED_CDB_VIEW is now an underscore in Oracle 12.2

Posted on July 21, 2017 by Mike.Dietrich Parameters Single-/Multitenant 5

Hiding information is not good - and exclude_seed_cdb_view is now an underscore in Oracle 12.2 I have received several questions from customers in the past weeks using Oracle Multitenant about the parameter exclude_seed_cdb_view.It magically disappeared and morphed into _exclude_seed_cdb_view since Oracle Database 12.2.0.1. To be fair, the exclude_seed_cdb_view is listed in the list of obsolete parameters in Oracle Database 12.2.0.1.

What is EXCLUDE_SEED_CDB_VIEW?

This parameter hides everything belonging to the PDB$SEED from the usual queries. For instance you ran a query against CDB_DATA_FILES but you won’t see the data files belonging to PDB$SEED in the result set. Whether this is good or bad, at least RMAN (Recovery Manager) worked always correctly.

But I’m …

Where do these large trace files come from in Oracle 12c?

Posted on October 20, 2015 by Mike.Dietrich Behavior Change Oracle Database 12c Parameters 5

Just had an observation about very large trace files on one of my customers I’m working with at the moment. When I write “very” I mean “VERY” as some grew over 10GB within a few hours.

Growth - Trace Files - (c) Mike Dietrich

The files contained a ton of such messages:

----- Cursor Obsoletion Dump sql_id=5p8a9d4017bq3 -----
Parent cursor obsoleted 1 time(s). maxchild=1024 basephd=00007FFB8AD45CB0 phd=00007FFB8AD45CB0

After doing a bit of research I came across this document and an explanation:

MOS Note:1955319.1;
Huge Trace Files Created Containing “—– Cursor Obsoletion Dump sql_id=%s —–“

Well, we introduced an Enhancement – via an unpublished bug (and I’d guess it is …

Underscore

Upgrade to 26ai may hang due to block checking events

What would you see?

Datapatch return codes and helpful queries

Datapatch return codes

New interesting underscores for more than 255 _FIX_CONTROLs

Why don’t you see them in ORAdiff?

Oracle 12.2 and higher: Set _cursor_obsolete_threshold to old default

What is _cursor_obsolete_threshold?

Drop a tablespace in a PDB with a Guaranteed Restore Point being active

Drop a tablespace in a PDB with a Guaranteed Restore Point being active

Remove Underscore Parameters not only before you upgrade

Remove Underscore Parameters not only before you upgrade

Why EXCLUDE_SEED_CDB_VIEW is now an underscore in Oracle 12.2

What is EXCLUDE_SEED_CDB_VIEW?

Where do these large trace files come from in Oracle 12c?

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