Troubleshooting

Jonathan Lewis's picture

Spatial space

One thing you (ought to) learn very early on in an Oracle career is that there are always cases you haven’t previously considered. It’s a feature that is frequently the downfall of “I found it on the internet” SQL.  Here’s one (heavily paraphrased) example that appeared on the OTN database forum a few days ago:

select table_name,round((blocks*8),2)||’kb’ “size” from user_tables where table_name = ‘MYTABLE';

select table_name,round((num_rows*avg_row_len/1024),2)||’kb’ “size” from user_tables where table_name = ‘MYTABLE';

The result from the first query is 704 kb,  the result from the second is 25.4 kb … fragmentation, rebuild, CTAS etc. etc.

Jonathan Lewis's picture

Bind Effects

A couple of days ago I highlighted an optimizer anomaly caused by the presence of an index with a descending column. This was a minor (unrelated) detail that appeared in a problem on OTN where the optimizer was using an index FULL scan when someone was expecting to see an index RANGE scan. My earlier posting supplies the SQL to create the table and indexes I used to model the problem – and in this posting I’ll explain the problem and answer the central question.

Jonathan Lewis's picture

FBI Bug reprise

I’ve just had cause to resurrect a blog note I wrote three years ago. The note says that an anomaly I discovered in 9.2.0.8 wasfixed in 10.2.0.3 – and this is true for the simple example in the posting; but a recent question on the OTN database forum has shown that the bug still appears in more complex cases.  Here’s some code to create a table and two indexes:

Jonathan Lewis's picture

count(*) – again !

Because you can never have enough of a good thing.

Here’s a thought – The optimizer doesn’t treat all constants equally.  No explanations, just read the code – execution plans at the end:

randolf.geist's picture

New Version Of XPLAN_ASH Utility

A new version 4.2 of the XPLAN_ASH utility is available for download.

As usual the latest version can be downloaded here.

There were no too significant changes in this release, mainly some new sections related to I/O figures were added.

One thing to note is that some of the sections in recent releases may require a linesize larger than 700, so the script's settings have been changed to 800. If you use corresponding settings for CMD.EXE under Windows for example you might have to adjust accordingly to prevent ugly line wrapping.

Here are the notes from the change log:

- New sections "Concurrent activity I/O Summary based on ASH" and "Concurrent activity I/O Summary per Instance based on ASH" to see the I/O activity summary for concurrent activity

oakroot's picture

[Oracle] Researching internal latch implementation (ksl_get_shared_latch, kslfre, kslgetsl_w) and crashing PMON

Introduction

Last week the DOAG 2014 conference took place in Nuremberg and it was a blast with a lot of useful presentations and especially great conversations and meet ups with Oracle friends. I had a nice talk about the Oracle latch implementation with a participant, who told me that his instance crashes every time, if he (manually) sets a shared latch in exclusive mode and tries to release it afterwards. It sounded really interesting as i have done this so many times without ever noticing such an issue. He also told me that this issue is reproducible at least on Oracle 11g R2 and 12c R1. I had no immediate answer or clue about the described issue and needed to research it furthermore.

Jonathan Lewis's picture

push_pred – evolution

Here’s a query (with a few hints to control how I want Oracle to run it) that demonstrates the difficulty of trying to solve problems by hinting (and the need to make sure you know where all your hinted code is):

Jonathan Lewis's picture

Parse Time

Here’s a little query I wrote some time ago to see where my time went while running a query. It’s nothing sophisticated, just one of those simple things you can do with v$active_session_history (or dba_hist_active_sess_history, if you don’t get to the crime scene in time).

Jonathan Lewis's picture

Line Numbers

One of the presentations I went to at the DOAG conference earlier on this month was called “PL/SQL Tuning, finding the perf. bottleneck with hierarchical profiler” by Radu Parvu from Finland. If you do a lot of PL/SQL programming and haven’t noticed the dbms_hprof package yet make sure you take a good look at it.

A peripheral question that came up at the end of the session asked about problems with line numbers in pl/sql procedures; why, when you get a run-time error, does the reported line number sometimes look wrong, and how do you find the right line. I can answer (or give at least one reason for) the first part, but not the second part; Julian Dontcheff had an answer for the second bit, but unfortunately I failed to take a note of it.

Jonathan Lewis's picture

Baselines

I’m not very keen on bending the rules on production systems, I’d prefer to do things that look as if they could have happened in a completely legal fashion, but sometimes it’s necessary to abuse the system and here’s an example to demonstrate the point. I’ve got a simple SQL statement consisting of nothing more than an eight table join where the optimizer (on the various versions I’ve tested, including 12c) examines 5,040 join orders (even though _optimizer_max_permutations is set to the default of 2,000 – and that might come as a little surprise if you thought you knew what that parameter was supposed to do):

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