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Quiz Night

I was setting up a few tests on a copy of recently when I made a mistake creating the table – I forgot to put in a couple of CAST() calls in the select list, so I just patched things up with a couple of “modify column” commands. Since I was planning to smash the table in all sorts of ways and it had taken me several minutes to create the data set (10 million rows) I decided to create a clean copy of the data so that I could just drop the original table and copy back the clean version – and after I’d done this I noticed something a little odd.

Here’s the code (cut down to just 10,000 rows), with a little output:

[Oracle] Trace back to responsible SQL or PL/SQL code for a particular (PGA) memory request by intercepting process with DTrace


This is just a short blog post about a simple DTrace script (dtrace_kghal_pga_code), that i recently wrote and published due to a PGA memory leak troubleshooting assignment. A client of mine noticed a major PGA memory increase after upgrading to Oracle 12c. The PL/SQL code did not change - just the database release. He already troubleshooted the issue with help of Tanel Poder's blog post "Oracle Memory Troubleshooting, Part 4: Drilling down into PGA memory usage with V$PROCESS_MEMORY_DETAIL" and identified the corresponding heap and allocation reason.

Trace file size

Here’s a convenient enhancement for tracing that came up on Twitter a few days ago – first in a tweet that I retweeted, then in a question from Christian Antognini based on this bit of the 12c Oracle documentation (opens in separate tab). The question was – does it work for you ?

The new description for max_dump_file_size says that for large enough values Oracle will split the file into multiple chunks of a few megabytes, using a suffix to identify the sequence of the chunks, keeping only the first chunk and the most recent chunks. Unfortunately this doesn’t seem to be true. However, prompted by Chris’ question I ran a quick query against the full parameter list looking for parameters with the word “trace” in their name:

Oracle XE 12c?

There was a post on Oracle-L asking about Oracle Express Edition (XE) 12c. I started to write a reply, but thought a blog post may be more appropriate.

Oracle XE 12c doesn’t exist yet, but people at OpenWorld 2015 confirmed they “plan” to have one. As always, no promises. So when will it arrive? Typically the XE version is put together based on the the first major patchset of release 2 of a version. So the kind of thing you might expect is,

  • : Released mid 2016 maybe.
  • : Released mid 2017 maybe.
  • XE team brought together: Some time after release.
  • XE Released: When it is built and stable.

Things to consider, based on stuff I’ve heard over the last few years.

Parallel Plans

This article was prompted by a pair of articles by Yasin Baskan of Oracle Corporation: PX Server Sets etc. and Multiple Parallelizers, plus a little extra prompting from a mistake that I made when reading the second of those two articles. The fact that I made a mistake is significant because, without it, I wouldn’t have created a model to check Yasin’s description of the parallel activity.

I want to examine the following query to find out the order of activity:

[Oracle] Understanding the Oracle code instrumentation (wait interface) - A deep dive into what is really measured


This blog post is inspired by a question from an attendee of Sigrid Keydana's DOAG 2015 conference session called "Raising the fetchsize, good or bad? Exploring memory management in Oracle JDBC 12c". Basically it was a question about what the wait event "SQL*Net more data to client" represents and what it really measures. In general you may use the following steps, if you don't know what a particular wait event means:


One of the questions that came up at the Optimizer Round Table this year was about minimizing the performance-related** hassle of upgrading from 11g to 12c. Dealing with changes in the optimizer is always an an interesting problem but in 12c this is made more challenging because of the automatic dynamic sampling that can introduce a significant amount of extra work at (hard) parse time, then generate SQL Directives, and finally generate extended (column group) statistics the next time you (or the automatic job) collect stats.

12c Scalar Subquery

Every version of the optimizer enhances existing mechanisms and introduces new features and 12c has introduced some of the most sophisticated transformation to date; in this note I want to demonstrate an enhancement to subquery unnesting that could give a significant performance boost to a certain query pattern but which might, unfortunately, result in worse performance.

Historically subquery unnesting turned subqueries (correlated or not) in the where clause into joins. In 12c subquery unnesting can also turn scalar subqueries in the select list into joins – we’ll discuss why this could be a good thing but might occasionally be a bad thing later on in the article, but let’s start with a test case.

Sample data.

In my demonstration I’m going to use three tables which, for convenience, are three clones of the same data.

Table Expansion

I’ve often found that while I’m investigating one Oracle feature I get waylaid by noticing anomalies in other parts of the code. I was caught by one of these events a little while ago while experimenting with the new ( Inmemory Columnar Store.  After reading a posting by Martin Bach I asked the question:

“If you have a partitioned table with a local index and one of the table partitions has been declared INMEMORY, would a query that could use that index be able to apply table expansion to produce a plan that did a tablescan on the in-memory partition and an indexed access path on the partitions that weren’t in-memory?”

Fedora 23 and Oracle 11gR2/12cR1

A few months ago I mentioned doing some Fedora 22 installations. At the time I did some pre-emptive installations on the Alpha release of Fedora 23 also.

Now the final release of Fedora 23 is out, I’ve run through the articles again to make sure things are all ship-shape.