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Showing posts from February, 2016

PDB saving state does not save its state on shutdown

When 12.1.0.1 came out, one of the gripes was that upon a CDB start, all the PDBs were in the mounted mode. The DBA had to open them manually, or use a database trigger to do that.

12.1.0.2 introduced SAVE STATE - according to the docs:
For example, if a PDB is in open read/write mode before the CDB is restarted, then the PDB is in open read/write mode after the CDB is restarted; if a PDB is in mounted mode before the CDB is restarted, then the PDB is in mounted mode after the CDB is restarted.
The trouble is that this is simply wrong, it does not work like this. Oracle has a table externalized as  DBA_PDB_SAVED_STATES and this stores the state. The table is updated only by the SAVE STATE command - and reflects the status when the SAVE STATE was issued, not when the database goes down.
It simply stores the open mode of the database and the CDB will open the database in this mode when the CDB opens. Lack of a row implies MOUNTED mode, i.e. the CDB won't do anything.
The row is dele…

Small addendum to the lying (Data Guard) broker

A friend of mine (Deiby Gómez) pointed me to an interesting article on MOS 1956103.1 - Warning: standby redo logs not configured for thread on db_unique_name/db_unique_name.

It essentially describes the same issue I described in
Don't trust the lying (Data Guard) broker - the newly created SRLs are not assigned to a particular thread and the VALIDATE command does not like it, although the standby is perfectly happy and will grab the SRLs as necessary, as it always did before 12c.

The Metalink note adds a solution - to assign the the SRLs to the threads manually during creation. The syntax is
alter database add standby logfile thread 1 group 1 'file spec' size .... This thus disables the auto-assign to the thread that needs it, but that should not matter. We usually size all the threads uniformly and assign enough SRLs to all of them, in other words we expect even distribution of SRLs to threads. Thus doing it manually is not a bad thing.

How many columns in a query

Everybody knows that the limit for number of columns in an Oracle table is 1000. It is actually limit of all columns in the table, including internal ones, virtual, unused but not yet dropped and so on.

But what is the limit for a query?

Let's start with a simple table, called many_columns. It has 1000 columns, all NUMBERs, to make things easy. Columns are named COLUMN_0001 to COLUMN_1000.

And I insert 1 row into the table:

insert into many_columns(COLUMN_0001) values (1); commit;
So what happens with an innocent query?

select m.*, n.* from many_columns m, many_columns n;
Well, nothing special - SQL*Plus is happy to return 2000 columns.

Obviously, there must an upper limit, right? At the very maximum, OCI specifies value for column count as ub2, i.e. max 65535.
However, SQL*Plus complains much sooner: the limit seems to be 8150. I added one more table - many_columns2 with just then columns. The first query to go over the limit, with 8151, fails with:

select m01.*, m02.*, m03.*, m04…

Docker machine - wonderful idea, too many bugs?

When doing various experiments with docker, I painfully realized that btrfs keeps a lot to be desired.

Wonderful idea, terrible user experience. First of all, df lies, and you are supposed to run btrfs balance often. Maybe it's because of the way docker uses it - it creates a ton of large images.
Eventually you touch all chunks and rebalance stops working completely. Now you desperately delete things, hoping to get chunk free and let rebalance get thins back to order. Or not - and you end up nuking the server and reinstalling.
Or you perhaps end up crashing up the server and the btrfs won't mount anymore...

So after going through 5 servers (OL7.1 in VBox), I moved onto docker-machine. Wonderful idea - and it does not use btrfs, yay!

However, it also has it's bugs... and pretty ugly. First of all, the latest stable boot2docker 1.9 has a kernel bug that causes Java process to become zombies and docker container won't finish. See https://github.com/docker/docker/issues/18…