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Histogram Upgrade – 2

While reading a blog post by Maria Colgan a couple of weeks ago I came across an observation about histograms that I’d not noticed before; worse still, it was a feature that seemed to make some “damage-limitation” advice I’d been giving for years a really bad idea! The threat appeared in these paragraphs:

Setting SIZE REPEAT ensures a histogram will only be created for any column that already has one. If the table is a partitioned table, repeat ensures a histogram will be created for a column that already has one on the global level.

What’s the down side to doing this?

The current number of buckets used in each histogram becomes the limit on the maximum number of buckets used for any histogram created in the future.

Oracle 12cR2: exchange partition deferred invalidation

In a previous post I introduced the new 12cR2 feature where some DDL operations can use the same rolling invalidation than what is done with dbms_stats. On tables, DDL deferred invalidation is available only for operations on partitions. Here is how it works for partition exchange.

Here is my session environment:

SQL> whenever sqlerror exit failure
SQL> alter session set nls_date_format='hh24:mi:ss';
Session altered.
SQL> alter session set session_cached_cursors=0;
Session altered.
SQL> alter session set optimizer_dynamic_sampling=0;
Session altered.
SQL> alter system set "_optimizer_invalidation_period"=5;
System SET altered.
SQL> show user
USER is "DEMO"

Virtualizing Big Data in the Cloud

I’m in sunny Phoenix this week at the Data Platforms 2017 Conference and looking forward to a break in the heat when I return to Colorado this evening.

As this event is for big data, I expected to present on how big data could benefit from virtualization, but was surprised to find that I didn’t have a lot of luck finding customers utilizing us for this reason, (yet).  As I’ve discussed in previous presentations, I was aware of what a “swiss army knife” virtualization is, resolving numerous issues, across a myriad of environments, yet often unidentified.

Parallelism

Headline – if you don’t want to read the note – the /*+ parallel(N) */ hint doesn’t mean a query will use parallel execution, even if there are enough parallel execution server processes to make it possible. The parallel(N) hint tells the optimizer to consider the cost of using parallel execution for each path that it examines, but ultimately the optimizer will still take the lowest cost path (bar the odd few special cases) and that path could turn out to be a serial path.

The likelihood of parallelism appearing for a given query changes across versions of Oracle so you can be fooled into thinking you’re seeing bugs as you test new versions but it’s (almost certainly) the same old rule being applied in different circumstances. Here’s an example – which I’ll start off on 11.2.0.4:

255 Again!

There are so many things that can go wrong when you start using tables with more than 255 columns – here’s one I discovered partly because I was thinking about a client requirement, partly because I had a vague memory of a change in behaviour in 12c and Stefan Koehler pointed me to a blog note by Sayan Malakshinov when I asked the Oak Table if anyone remembered seeing the relevant note. Enough of the roundabout route, I’m going to start with a bit of code to create a table, stick a row in it, then update that row:

255 columns

This is one of my “black hole” articles – I drafted it six months ago, but forgot to publish it.

Insider View of a Delphix VDB Rewind

I love questions-  They give me something to write about that I don’t have to come up with from my experience or challenges…:)

So in my last post, Paul asked:

I am not sure what happens to the other changes which happened while the release was happening? Presumably they are also lost? Presumably the database has to go down while the data is reverted?

The Setup

In our scenario to answer this question, I’m going to perform the following on the VEmp_826 virtualized database:

Quantum Space

Here’s a not very serious note that makes a serious point.  I’ve got a small tablespace made up of 4 files, and here’s a little report I can run against the data dictionary for that tablespace:


select 'File space' What, nvl(sum(user_bytes)/1048576,0) MB from dba_data_files where tablespace_name = 'LOB_TEST'
union all
select 'Free space',      nvl(sum(bytes/1048576),0)         from dba_free_space where tablespace_name = 'LOB_TEST'
union all
select 'Extents',         nvl(sum(bytes/1048576),0)         from dba_extents    where tablespace_name = 'LOB_TEST'
union all
select 'Segments',        nvl(sum(bytes/1048576),0)         from dba_segments   where tablespace_name = 'LOB_TEST'
;

The name of the tablespace isn’t significant – it happens to be a tablespace I created to do some tests relating to space allocation with securefile LOBs, and it’s been hanging around ever since.

Announcing SLOB 2.4! Integrated Short Scans and Cloud (DBaaS) Support, and More.

This post is to announce the release of SLOB 2.4!

VERSION

SLOB 2.4.0. Release notes (PDF): Click Here.

WHERE TO GET THE BITS

As always, please visit the SLOB Resources page. Click Here.

What is in a transportable tablespace dumpfile?

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 show what is inside the dumpfile when using Data Pump to export the metadata. Here is the idea.

expdp transport_tablespaces

Here is how we export metadata with Data Pump for transportable tablespaces.