statistics

Jonathan Lewis's picture

Clustering_Factor

Here’s another little note on the clustering_factor for an index and the table preference table_cached_blocks that can be set with a call to dbms_stats.set_table_prefs(). I might be repeating a point that someone made in a comment on an older posting but if that’s the case I can’t find the comment at present, and it’s worth its own posting anyway.

davidkurtz's picture

Managing Cost-Based Optimizer Statistics for PeopleSoft

I gave this presentation to UKOUG PeopleSoft Roadshow 2018

PeopleSoft presents some special challenges when it comes to collecting and maintaining the object statistics used by the cost-based optimizer.

Jonathan Lewis's picture

Bitmap Join Indexes

I’ve been prompted by a recent question on the ODC database forum to revisit a note I wrote nearly five years ago about bitmap join indexes and their failure to help with join cardinalities. At the time I made a couple of unsupported claims and suggestions without supplying any justification or proof. Today’s article finally fills that gap.

Jonathan Lewis's picture

exp catch

No-one should be using exp/imp to export and import data any more, they should be using the datapump equivalents expdp/impdp – but if you’re on an older (pre-12c) version of Oracle and still using exp/imp to do things like moving tables with their production statistics over to test systems then be careful that you don’t fall into an obsolescence trap when you finally upgrade to 12c (or Oracle 18).

exp/imp will mess up some of your histograms if you’re still using them to move tables/statistics in 12c.

Jonathan Lewis's picture

Column Groups

There’s a question on the ODC database forum about column groups that throws up an interesting side point. The OP is looking at a query like the following and asking about which column groups might help the optimizer get the best plan:

Franck Pachot's picture

V$MYSTAT delta values

Here is a little script I use from time to time to look at V$MYSTAT values and displaying on one line a set of statistics with their delta value between two calls.

The first script, _mystat_init.sql, initializes the variables. The second one displays the values, such as:

SQL> @ _mystat_diff.sql
 
db block changes redo size undo change vector size redo entries
---------------- ---------------- ----------------------- ----------------
57,371 15,445,852 6,111,608 37,709

Jonathan Lewis's picture

Histogram Threat

Have you ever seen a result like this:


SQL> select sql_id, count(*) from V$sql group by sql_id having count(*) > 1000;

SQL_ID		COUNT(*)
------------- ----------
1dbzmt8gpg8x7	   30516

A client of mine who had recently upgraded to 12.2.0.1 RAC, using DRCP (database resident connection pooling) for an application using PHP was seeing exactly this type of behaviour for a small number of very simple SQL statements and wanted to find out what was going on because they were also seeing an undesirable level of contention in the library cache when the system load increased.

In this note I just want to highlight a particular detail of their problem – with an example – showing how easily histograms can introduce problems if you don’t keep an eye out for the dangers.

One of their queries really was as simple as this:

Jonathan Lewis's picture

Histograms

I had a sudden urge to go a bit meta – so here’s a relative frequency histogram of my observations of the general use of histograms in Oracle:

 

Jonathan Lewis's picture

Column Stats

I’ve made several comments in the past about the need for being selective when gathering objects statistics with particular reference to the trade-offs when creating histograms. With Oracle 12c it’s now reasonably safe (as far as I’m concerned) to set a method_opt as a table preference that identifies columns where you expect to see Frequency or (pace the buggy behaviour described in a recent post) a Top-N histograms. The biggest problem I have is that I keep forgetting the exact syntax I need – so I’ve written this note more as a reminder to myself than anything else.

Jonathan Lewis's picture

Histogram Hassle

I came across a simple performance problem recently that ended up highlighting a problem with the 12c hybrid histogram algorithm. It was a problem that I had mentioned in passing a few years ago, but only in the context of Top-N histograms and without paying attention to the consequences. In fact I should have noticed the same threat in a recent article by Maria Colgan that mentioned the problems introduced in 12c by the option “for all columns size repeat”.

So here’s the context (note – all numbers used in this example are approximations to make the arithmetic obvious).  The client had a query with a predicate like the follwing:

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