A recent question on the OTN database forum:
Can any one please point to me a document or a way to calculate the average number of rows per block in oralce 10.2.0.3
One answer would be to collect stats and then approximate as block / avg_row_len – although you have to worry about things like row overheads, the row directory, and block overheads before you can be sure you’ve got it right. On top of this, the average might not be too helpful anyway. So here’s another (not necessarily fast) option that gives you more information about the blocks that have any rows in them (I picked the source$ table from a 10g system because source$ is often good for some extreme behaviour).
From time to time I see people asking how they can check how far their system has got in it’s call to collect stats. The question is often a little ambiguous – they know which call to dbms_stats they’ve used, so they know whether they’re trying to monitor stats collection against a single table (taking advantage of v$session_longops springs to mind) or against an entire schema.
Here’s one simple-minded approach that I whipped up a few years ago – it’s possible you could do better with the latest versions of dbms_stats. Its purpose is simply to give you some way of checking what work dbms_stats needs to do (in this example, for a schema), so that you can check how much has been done and how much remains. The basic method can be modified in various ways to match your tastes.
I wrote an article about system statistics / CPU Costing for Oracle magazine a few years ago – and last week I realised that I’ve never supplied a link to it in the notes and comments I’ve made about system statistics. So I’ve just run a search through the Oracle website trying to find it – and discovered that it’s no longer available. Apparently the editors have decided that any technical articles over a certain age should be withdrawn in case they are out of date and misleading. (Clearly they’ve read my blog on trust – I wish the people maintaining Metalink would do the same as the magazine editors – but they probably have a much larger volume to worry about).
However, I have discovered translations of the article in Russian, Korean and Chinese – so if you can read any of these languages, you might want to take a look at them before they disappear too.
If you want an original English version – dated April 2004, which is when I sent it in to Oracle Magazine, and before it underwent some editing – I’ve posted it as a pdf file.
[More on System Statistics]
In an earlier post on frequency histograms I described how Oracle creates an approximate histogram when dealing with character columns, and warned you that the strategy could lead to a couple of anomalies if you were unlucky. I’ve already published a note about one such anomaly that can occur with fairly long character strings, this note describes another anomaly that could appear in less extreme cases. Again, we start by constructing a data set.
In an earlier post on frequency histograms I described how Oracle creates an approximate histogram when dealing with character columns, and warned you that the strategy could lead to a couple of anomalies if you were unlucky. This note describes one such anomaly. We start with a slightly curious data set:
In an earlier note on interpreting the content of frequency histograms I made a throwaway comment about the extra complexity of interpreting frequency histograms on character-based columns. This note starts to examine some of the complications.
The driving problem behind character columns is that they can get quite large – up to 4,000 bytes – so the content of an “accurate histogram” could become quite large, and Oracle seems to have taken a strategic decision (at some point in history) to minimise this storage. As a result we can see an algorithm that works roughly as follows:
Here’s the output I get from querying dba_tab_histograms for a column that has been given a frequency histogram by a call to dbms_stats.gather_table_stats().
I find it convenient occasionally to “translate” a frequency histogram into a report of the underlying data (sometimes to see how closely the histogram matches the real data). To demonstrate the type of query I use I’ve created a data set with a small number of distinct values and generated a frequency histogram on the data set. This is what the data and histogram look like:
In case you don’t follow the link to Martin Widlake’s blog (see right) very often, he’s done a couple of recent posts on dba_tab_modifications that are worth reading.
(And I’ve just discovered the ‘gutter=”false”;’ option for the ‘sourcecode’ tag in one of the comments on Martin’s blog – and that’s also a helpful feature.)
If you’ve run the scripts from Analyze This, I hope you found that the query gave you two different execution plans. This is my output from the test (with a little cosmetic tidying):
Here’s a little script I wrote a few years ago to make a point about using the dbms_stats package. I’ve just re-run it on 10.2.0.3 to see if it still behaves the way it used to – and it does. If you want to be just a little bit baffled, set up a database with an 8KB blocks size, a tablespace that is locally managed, uniform extent size of 1MB, using freelist management, then run the script:
I got an email a few days ago asking me how to create a “height-balanced” histogram using the dbms_stats.set_column_stats() procedure.
I posted an example of creating a frequency histogram a few months ago – and it doesn’t take much to turn this into an example for the height-balanced type; the key change apppears in three critical lines.
It’s possible to spend ages talking about the best ways of collecting, or creating, statistics on partitioned tables.
The possible strategies for maintaining partitioned tables, (exchange partition, split partition, drop partition etc.) the types of partitioning available, and the way that the optimizer plays with the stats as you do so, have kept changing over the years, and I’ve got a large set of examples designed to test what happens to the stats as you do things to the table – but it’s impossible to keep it up to date.
Doug Burns is writing a series of articles about the trials, tribulations, and successes about partitioned tables and statistics. The series was well worth reading and will give you an insight into the problems you may have to address, so I’ve produced a catalogue to make it easy to visit the individual chapters in order. Make sure you also read the comments and related links.
- Part 1 – In which we see a simple example and do a default stats collection
- Part 2 – In which we consider Global Stats
- Part 3 – In which subpartitions and aggregation cause problems
- Part 4 – In which our hero fights his way through stats aggregation woes
- Part 5 - In which we encounter a partition exchange
- Part 6a – In which we start to use dbms_stats.copy_table_stats()
- Part 6b – In which we see how reputable individuals handle their mistakes
- Part 6c – In which we hear about 10.2.0.5 and lots of bugs
- Part 6d – In which we revisit earlier errors and discuss the benefit of discussion
- Part 6e – In which we revisit earlier problem again and talk about a bug.
- Part 7 – Not in the original series, but an interesting (slow) experience in 11g
I thought I’d collate a few other items on partition stats and optimizer behaviour - mainly from Randolf Geist’s blog:
And one from Kerry Osbourne – which lists a new granularity option, and a patch for 10.2.0.4
- Feb 2009: Maintaining statistics on a large partitioned table. (See also Metalink Doc ID: 6526370.8)
A couple (as pdf files) from David Kurtz, with a particular view to optimising Peoplesoft.
And an investigation into an oddity with the optimizer when using partitioned indexes
- Feb 2011: Jokes of the CBO with local indexes (10.2.0.4, 22.214.171.124)
In a blog that’s now three years old I pointed out the issue of the automatic stats collection job and the way it could affect performance by changing object-level statistics behind your back. There’s another reason it could have an annoying side effect, though – it actually takes time to collect statistics.
The automatic job is, by default, allowed to run from 10:00 pm to 6:00 am on weekdays and for even longer over the weekend – and the time it needs to complete can vary dramatically from day to day. So it’s nice to have a little script to cross-check when it started and how long it ran. All you have to do is query the view dba_scheduler_job_run_details:
A few days ago I did a presentation on SQL Server. This probably sounds a little strange given my status as an Oracle specialist – but the nice people at Microsoft asked me if I would contribute to one of their seminars so I downloaded and installed the 180 day free trial copy of the Enterprise version, then downloaded the “Books Online” manuals and started to play.
It was an interesting experience – and I think the audience (and organisers) found my presentation useful. The title was “What the Enterprise needs in an RDBMS” – and that’s something I do know about – and the presentation was about whether or not you could find everything you needed in SQL Server 2008, where you’d have to look in the manuals, and supplementary questions you’d have to ask.