Infrastructure

Jonathan Lewis's picture

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 for columns} from v$session where sid = to_number(sys_context('userenv','sid'));

Unfortunately that one little statement hides two anomalies – which you can see in the execution plan. Here’s a demonstration cut from an SQL*Plus session running under 19.3.0.0:

Jonathan Lewis's picture

My SID

Here’s a little note that’s been hanging around as a draft for more than eight years according to the OTN (as it was) posting that prompted me to start writing it. At the time there were still plenty of people using Oracle 10g. so the question didn’t seem entirely inappropriate:

On 10g R2 when I open a sqlplus session how can I know my session SID ? I’m not DBA then can not open as sysdba and query v$session.

Jonathan Lewis's picture

Resumable

There are two questions about temporary space that appear fairly regularly on the various Oracle forums. One is of the form:

From time to time my temporary tablespace grows enormously (and has to be shrunk), how do I find what’s making this happen?

The other follows the more basic pattern:

My process sometimes crashes with Oracle error: “ORA-01652: unable to extend temp segment by %n in tablespace %s” how do I stop this happening?

Before moving on to the topic of the blog, it’s worth pointing out two things about the second question:

Jonathan Lewis's picture

Trace Files

A recent blog note by Martin Berger about reading trace files in 12.2 poped up in my twitter timeline yesterday and reminded me of a script I wrote a while ago to create a simple view I could query to read the tracefile generated by the current session while the session was still connected. You either have to create the view and a public synonym through the SYS schema, or you have to use the SYS schema to grant select privileges on several dynamic performance views to the user to allow the user to create the view in the user’s schema. For my scratch database I tend to create the view in the SYS schema.

Script to be run by SYS:

Jonathan Lewis's picture

_cursor_obsolete_threshold

At the recent Trivadis Performance Days in Zurich, Chris Antognini answered a question that had been bugging me for some time. Why would Oracle want to set the default value of _cursor_obsolete_threshold to a value like 8192 in 12.2 ?

In 11.2.0.3 the parameter was introduced with the default value 100; then in 11.2.0.4, continuing into 12.1, the default value increased to 1,024 – what possible reason could anyone have for thinking that 8192 was a good idea ?

The answer is PDBs – specifically the much larger number of PDBs a single CBD can (theoretically) support in 12.2.

In fact a few comments, and the following specific explanation, are available on MoS in Doc ID 2431353.1 “High Version Counts For SQL Statements (>1024) Post Upgrade To 12.2 and Above Causing Database Slow Performance”:

Jonathan Lewis's picture

Little sleeps

A peripheral question in a recent comment (made in response to me asking whether a loop had been written with a sleep time of 1/100th or 1/1000th of a second) asked “How do you sleep for 1/1000th of a second in pure PL/SQL?”

The answer starts with “How pure is pure ?” Here’s a “pure” PL/SQL solution that “cheats” by calling one of the routines in Oracle’s built-in Java library:

Jonathan Lewis's picture

Nologging

Bobby Durrett recently published a note about estimating the volume of non-logged blocks written by an instance with the aim of getting some idea of the extra redo that would be generated if a database were switched to “force logging”.

Since my most recent blog notes have included various extracts and summaries from the symbolic dumps of redo logs it occurred to me that another strategy for generating the same information would be to dump the redo generated by Oracle when it wanted to log some information about non-logged blocks. This may sound like a contradiction, of course, but it’s the difference between data and meta-data: if Oracle wants to write data blocks to disc without logging their contents it needs to write a note into the redo log saying “there is no log of the contents of these blocks”.

Jonathan Lewis's picture

Update restarts

Somewhere I think I’ve published a note about an anomaly that’s been bugging me since at least Oracle 10g – but if it is somewhere on the Internet it’s hiding itself very well and I can’t find it, though I have managed to find a few scripts on my laptop that make a casual reference to the side effects of the provlem. [Ed: a tweet from Timur Ahkmadeev has identified a conversation in the comments on an older post that could explain why I thought I’d written something about the issue.]

Anyway, I’ve decided to create some new code and write the article (all over again, maybe). The problem is a strange overhead that can appear when you do a simple but large update driving off a tablescan.

Jonathan Lewis's picture

Quiz Night

Upgrades cause surprises – here’s a pair of results from a model that I constructed more than 15 years ago, and ran today on 12.2, then modified and ran again, then ran on 11.2.0.4, then on 12.1.0.2. It’s very simple, I just create a table, gather stats, then update every row.

Jonathan Lewis's picture

sql_patch

This note is a short follow-up to a note I wrote some time ago about validating foreign key constraints where I examined the type of SQL Oracle generates internally to do the validation between parent and child tables.  In that article I suggested (before testing) that you could create an SQL patch for the generated SQL to over-ride the plan taken by Oracle – a plan dictated to some extent by hints (including a “deprecated” ordered hint) embedded in the code. I did say that the strategy might not work for SQL optimised by SYS, but it turned out that it did.

Here’s a little script I ran to test a few variations on the theme:

To prevent automated spam submissions leave this field empty.
Syndicate content