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Statspack

Fixed Stats

There are quite a lot of systems around the world that aren’t using the AWR (automatic workload repository) and ASH (active session history) tools to help them with trouble shooting because of the licensing requirement – so I’m still finding plenty of sites that are using Statspack and I recently came across a little oddity at one of these sites that I hadn’t noticed before: one of the Statspack snapshot statements was appearing fairly regularly in the Statspack report under the “SQL Ordered by Elapsed Time” section – even when the application had been rather busy and had generated lots of other work that was being reported. It was the following statement – the collection of file-level statistics:

Table Scans

It’s amazing how easy it is to interpret a number incorrectly until the point comes where you have to look at it closely – and then you realise that there was a lot more to the number than your initial casual assumption, and you would have realised it all along if you’d ever needed to think about it before.

Here’s a little case in point. I have a simple (i.e. non-partitioned) heap table t1 which is basically a clone of the view dba_segments, and I’ve just connected to Oracle through an SQL*Plus session then run a couple of SQL statements. The following is a continuous log of my activity:

Lock Time

Here’s a little detail I was forced to re-learn yesterday; it’s one of those things where it’s easy to say “yes, obviously” AFTER you’ve had it explained so I’m going to start by posing it as a question. Here are two samples of PL/SQL that using locking to handle a simple synchronisation mechanism; one uses a table as an object that can be locked, the other uses Oracle’s dbms_lock package. I’ve posted the code for each fragment, and a sample of what you see in v$lock if two sessions execute the code one after the other:

Table locking – the second session to run this code will wait for the first session to commit or rollback:

12c Statspack Hack

Starting from a comment on an old statspack/AWR page, with a near-simultaneous thread appearing on OTN, (do read both) here’s a quick summary of getting statspack onto 12c with containers. (For non-container databases it’s a standard install).

Option 1 – completely valid

At present, you have to install the perfstat account on a pluggable database if you want to do it legally. On the plus side this means you could install it once then clone, unplug, and re-plug it elsewhere – though you might have to play around each time enabling a new statspack job.

Transactions 2

Here’s a little follow-on from Friday’s posting. I’ll start it off as a quiz, and follow up tomorrow with an explanation of the results (though someone will probably have given the correct solution by then anyway).

I have a simple heap table t1(id number(6,0), n1 number, v1 varchar2(10), padding varchar2(100)). The primary key is the id column, and the table holds 3,000 rows where id takes the values from 1 to 3,000. There are no other indexes. (I’d show you the code, but I don’t want to make it too easy to run the code, I want you to try to work it out in your heads).

I run the following pl/sql block.

Transactions

It’s very easy to get a lot of information from an AWR (or Statspack) report – provided you remember what all the numbers represent. From time to time I find that someone asks me a question about some statistic and my mind goes completely blank about the exact interpretation; but fortunately it’s always possible to cross check because so many of the statistics are cross-linked. Here’s an example of a brief mental block I ran into a few days ago – I thought I knew the answer, but realised that I wasn’t 100% sure that my memory was correct:

In this Load Profile (for an AWR report of 60.25 minutes), what does that Transactions figure actually represent ?

Analysing Statspack 13

A recent (Jan 2013) post on the OTN database forum reported a performance problem on Oracle 9.2.0.6 (so no AWR), and posted a complete statspack report to one of the public file-sharing sites. It’s been some time since I did a quick run through the highlights of trouble-shooting with statspack, so I’ve picked out a few points from this one to comment on.

As usual, although this specific report is Statspack, the same analysis would have gone into looking at a more modern AWR report, although I will make a couple of comments at the end about the extra material that would have been available by default with the AWR report that would have helped us help the OP.

Statspack quiz

I’ve a level 5 Statspack report from a real production 11.2.0.2 database with the following Top 5 Timed events section:

Load profile

I like Load profile section of Statspack or AWR reports (who doesn’t). It’s short and gives a brief understanding of what kind of work a database does. But what if you don’t have an access to Statspack or AWR but still want to see something similar? It’s possible to use V$SYSMETRIC to get this numbers for last 60 or 15 seconds. I wanted to write a script to do this for a long time. Here it is.

Irrational Ratios

I’ve pointed out in the past how bad the Instance Efficiency ratios are in highlighting a performance problem. Here’s a recent example from OTN repeating the point. The question, paraphrased, was:

After going through AWR reports (Instance Efficiency Percentages) I observed they have low Execute to Parse % but high Soft Parse %.
Please share if you had faced such issue and any suggestions to solve this

The first problem with this question is that ratios hide information.
The second problem is that we don’t know what values the OP considers to be “low” or “high” in this context.
The third is that there’s no reason why any such juxtaposition of ratios should implicitly mean there’s a problem.