The holistic SQL tuning series

I did a set of articles for Oracle Magazine on a more holistic view of SQL tuning. What do I mean by “holistic”? It was a reflection of a common problem that I see when questions come into AskTOM, or when people in the community approach me at conferences, namely, there is an inclination to dive straight into the deepest levels of the tuning exercise:

  • “What index should I create?”
  • “Should I increase the (more...)

OGB Appreciation Day : Exadata X8M

What is OGB Appreciation Day?

The Oracle Groundbreakers (OGB) Appreciation Day formally known as OTN Appreciation Day and ODC Appreciation Day, is a great initiative by Tim Hall aka Oracle-Base.com.  Where we take the opportunity to say thanks to the Oracle Community which includes but not limited to ACEs, Java Champions, Ambassadors and all those who have the Groundbreakers spirit #ThanksOGB 🙂

I wonder what will be the name will be next year 😉

(more...)

v$session

Here’s an odd, and unpleasant, detail about querying v$session in the “most obvious” way. (And if you were wondering what made me resurrect and complete a draft on “my session id” a couple of days ago, this posting is the reason). Specifically if you want to select some information for your own session from v$session the query you’re likely to use in any recent version of Oracle will probably be of the form:


select {list  (more...)

The definition of proof

One of the pieces of advice that I often see on the ‘net is that undo space is somehow this incredibly precious thing, and as a consequence, one should always keep the amount of uncommitted changes in the database to a small size.

Personally I think that is baloney (Ed-in reality, as an Australian I have a slightly more powerful choice of term, but lets keep things PG-rated 🙂). But when I recently challenged (more...)

Little things worth knowing: keeping enq: TM enqueue at bay during direct path inserts

Direct path inserts are commonly found in processing where data are shifted from one source to another. There are many permutations of the theme, this post details the way SQL Loader (sqlldr) behaves.

I have previously written about sqlldr and concurrent direct path inserts. The behaviour for releases <= 12.1 is described in the first post, the most welcome changes in 12.2 went into the second post. Since the fastest way of (more...)

Online Redo Log Switch Frequency Map

A query I find myself often running is the online redo log switch frequency map query, which queries the v$log_history/gv$log_history (for cluster databases) view and show the historical log switch frequency.

Why you might ask? Well it’s important to see how frequent log switches are occurring as Oracle’s rule of thumb is to not switch more then 3 logs per hour (20 minutes of redo) at peak DML activity to prevent excessive checkpoints.  The (more...)

RMAN Incremental Differential vs Cumulative & Demo

This blog post is part of the “RMAN Back to Basics” series, which can be found here.

Differential Incremental Backups (default)

A differential incremental backup, backs up all the blocks that have changed after the most recent incremental backup, which can be either level 1 or 0.

RMAN determines which is the most recent level 1 backup and then backs up all the blocks changed after that backup, if no level 1 then all the (more...)

RMAN Incremental with Block Change Tracking & Demo

This blog post is part of the “RMAN Back to Basics” series, which can be found here.

Block Change Tracking

Block change tracking (BCT) improves incremental backup performance by recording the changed blocks in each datafile in the block change tracking file, thus avoiding the need to scan every block in the datafile for changes.  Only applicable for level 1 backups.  Block change tracking file is 1/30,000 the size of the data blocks being tracked, (more...)

Indexing Null Values – Part 2

In the previous post I've demonstrated that Oracle has some problems to make efficient use of B*Tree indexes if an IS NULL condition is followed by IN / OR predicates also covered by the same index - the predicates following are not used to navigate the index structure efficiently but are applied as filters on all index entries identified by the IS NULL.

In this part I'll show what results I got when repeating the (more...)

Indexing Null Values – Part 1

Indexing null values in Oracle is something that has been written about a lot in the past already. Nowadays it should be common knowledge that Oracle B*Tree indexes don't index entries that are entirely null, but it's possible to include null values in B*Tree indexes when combining them with something guaranteed to be non-null, be it another column or simply a constant expression.

Jonathan Lewis not too long ago published a note that showed an (more...)

When views causes havoc

Views are great. They simplify design, makes code look more elegant and hides complexity. They also enables reuse by putting complex code in just one place instead of in every accessing piece of code.

But…

There once was a large project that has been churning away for a long time. The performance of having more than one user on the system was horrendous. But it was chalked up to misconfiguration.

I am asked to spend (more...)

PeopleSoft Administrator Podcast: #183 – Effective Performance Monitoring

I recently recorded a podcast with Dan Iverson and Kyle Benson for the PeopleSoft Administrator Podcast, this time about instrumentation, monitoring the performance of PeopleSoft system, and Performance Monitor.  There is also just a little about cursor sharing.

(3 May 2019) #183 – Effective Performance Monitoring

You can listen to the podcast on psadmin.io, or subscribe with your favourite podcast player, or in iTunes.

I/O Benchmark Minor Update

I've recently published a new version 1.03 of the I/O benchmark scripts on my github repository (ideally pick the IO_BENCHMARK.ZIP containing all the scripts). The original post including some more instructions can be found here, and there is also a video on my Youtube channel explaining how to use the benchmark scripts.

The main change is a new version of the "Write IOPS" benchmark that should scale much better than the (more...)

Bloom Filter Efficiency And Cardinality Estimates

I've recently came across an interesting observation I've not seen documented yet, so I'm publishing a simple example here to demonstrate the issue.

In principle it looks like that the efficiency of Bloom Filter operations are dependent on the cardinality estimates. This means that in particular cardinality under-estimates of the optimizer can make a dramatic difference how efficient a corresponding Bloom Filter operation based on such a cardinality estimate will work at runtime. Since Bloom (more...)

Parallel Madness

usain-bolt-1I’ve noticed at a few clients with data warehouses recently that the Developers and, upon occasion, Business Users have a real fondness for hinting the SQL they are producing with one particular hint. PARALLEL.

As any fule kno, this IS the magic go-faster hint. PARALLEL(2) is obviously twice as fast as serial execution. PARALLEL(4) is amazing and PARALLEL(64) like Usain Bolt on Red Bull.

The problem is that, like all database features, parallel query (more...)

Speeding up Initial data load for Oracle to PostgreSQL using Goldengate and copy command

Original Post can be viewed at Speeding up Initial data load for Oracle to PostgreSQL using Goldengate and copy command

Oracle Goldengate supports Oracle to PostgreSQL migrations by supporting PostgreSQL as a target database, though reverse migration i.e PostgreSQL to Oracle is not supported. One of the key aspect of these database migrations is initial data load phase where full tables data have to copied to the target datastore. This can be a time (more...)

Lies, darn lies and sampling bias

Sampling profiling is very powerful technique widely used across various platforms for identifying hot code (execution bottlenecks).

In Java world sampling profiling (thread stack sampling to be precise) is supported by every serious profiler.

While being powerful and very handy in practice, sampling has well known weakness – sampling bias. It is real and well-known problem, though its practical impact is often being over exaggerated.

A picture is worth a thousand of words, so let (more...)

PostgreSQL – LWLock:multixact_offset

On one of the PostgreSQL db , the active sessions were waiting on IO:SLRURead and LWLock:multixact_offset causing application latency.

Multioffset

As per PostgreSQL doc, SLRURead is waiting for a read of an SLRU page and multixact_offset is waiting for I/O on a multixact offset buffer.

Diagnosing the issue took some time so cut to the chase

1. Identified the SQLs, relation (and its size) for which these wait events were high. These table had ~1500 DMLs (more...)

A Performance Dashboard for Apache Spark

Topic: This post dives into the steps for deploying and using a performance dashboard for Apache Spark, using Spark metrics system instrumentation, InfluxDB and Grafana.

What problem does it solve: The dashboard can provide important insights for performance troubleshooting and online monitoring of Apache Spark workloads. In particular when running Spark applications in a distributed environment (Kubernetes, YARN, etc.).

Context: Familiarity with Spark architecture built-in instrumentation, notably the Web UI. This discussion also overlaps (more...)

shared resources

Some days ago I had one of these calls from an application colleague:
The database was slow yesterday, can you please check what's the problem?
Of course, I had some short discussion if he really means the DB or should it be called the application is slow. Also some other questions needed to be asked first, e.g. if it's a response time or throughput issue, when it was "good" last time, what "bad" and (more...)