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Parallel Execution

Chinar Aliyev's Blog

Chinar Aliyev has recently started to pick up on several of my blog posts regarding Parallel Execution and the corresponding new features introduced in Oracle 12c.

It is good to see that obviously Oracle has since then improved some of these and added new ones as well.

Here are some links to the corresponding posts:

New automatic Parallel Outer Join Null Handling in 18c

Improvements regarding automatic parallel distribution skew handling in 18c

Chinar has also put some more thoughts on the HASH JOIN BUFFERED operation:

Hash Optimisation-

Franck Pachot did an interesting presentation at the OBUG (Belgium user group) Tech Days showing how to use one of the O/S debug/trace tools to step through the function calls that Oracle made during different types of joins. This prompted me to ask him a question about a possible optimisation of hash joins as follows:

The hash join operation creates an in-memory hash table from the rowsource produced by its first child operation then probes the hash table with rows from the row source produced by the second child operation; but if there are no rows in the first row source then there’s no need to acquire rows from the second row source, so Oracle doesn’t call the second child operation.

DML Tablescans

This note is a follow-up to a recent comment a blog note about Row Migration:

So I wonder what is the difference between the two, parallel dml and serial dml with parallel scan, which makes them behave differently while working with migrated rows. Why might the strategy of serial dml with parallel scan case not work in parallel dml case? I am going to make a service request to get some clarifications but maybe I miss something obvious?

The comment also referenced a couple of MoS notes:

QC vs. PX

One last post before closing down for the Christmas break.

Here’s a little puzzle with a remarkably easy and obvious solution that Ivica Arsov presented at the UKOUG Tech2018 conference. It’s a brilliant little puzzle that makes a very important point, because it reminded me that most problems are easy and obvious only after you’ve seen them at least once. If you’ve done a load of testing and investigation into something it’s easy to forget that there may be many scenarios you haven’t even thought of testing – so when you see the next puzzle your mind follows all the things you’ve done previously and doesn’t think that you might be looking at something new.

I don’t know (yet)

Here’s a question that came to mind while reading a recent question on the OTN database forum. It’s a question to which I don’t know the answer and, at present, I don’t really want to bother modelling at present – although if I were on a customer site and this looked like a likely explanation for a performance anomaly it’s the sort of thing I would create a model for.

Oracle Parallel Execution Deep Dive Session

Here is a recording available on Youtube of a session I did a while ago, covering how to understand the essentials of Oracle Parallel Execution and how to read the corresponding execution plans.

Parallel_index hint

Prompted by a recent OTN posting I’ve dug out from my library the following demonstration of an anomalty with the parallel_index() hint. This note is a warning about  how little we understand hints and what they’re supposed to mean, and how we can be caught out by an upgrade. We’ll start with a data set which, to match a comment made in the origina posting rather than being a necessity for the demonstration, has an index that I’ve manipulated to be larger than the underlying table:

New Version Of XPLAN_ASH Utility

A new version 4.23 of the XPLAN_ASH utility is available for download.

As usual the latest version can be downloaded here.

This version comes only with minor changes, see the change log below.

Here are the notes from the change log:

- Finally corrected the very old and wrong description of "wait times" in the script comments, where it was talking about "in-flight" wait events but that is not correct. ASH performs a "fix-up" of the last 255 samples or so and updates them with the time waited, so these wait events are not "in-flight"

- Removed some of the clean up code added in 4.22 to the beginning of the script, because it doesn't really help much but spooled script output always contained these error messages about non-existent column definitions being cleared

Uniquely parallel

Here’s a surprising (to me) execution plan from 12.1.0.2 – parallel execution to find one row in a table using a unique scan of a unique index – produced by running the following script (data creation SQL to follow):

dbms_xplan

My favourite format options for dbms_xplan.display_cursor().

This is another of those posts where I tell you about something that I’ve frequently mentioned but never documented explicitly as a good (or, at least, convenient) idea. It also another example of how easy it is to tell half the story most of the time when someone asks a “simple” question.