This is a quick response to a question on an old blog post asking how you can adjust the high value if you’ve already got a height-balanced histogram in place. It’s possible that someone will come up with a tidier method, but this was just a quick sample I created and tested on 22.214.171.124 in a few minutes. (Note – this is specifically for height-balanced histograms, and it’s not appropriate for 12c which has introduced hybrid histograms that will require me to modify my “histogram faking” code a little).
Like the recent article on deleting histograms this is another draft that I rediscovered while searching for some notes I had written on a different topic – so I’ve finally finished it off and published it.
Here’s a quirky little detail of extended stats that came up in an OTN thread earlier on this week [ed: actually 8th Jan 2014]. When you create column group stats, Oracle uses an undocumented function sys_op_combined_hash() to create a hash value, and if you gather simple stats on the column (i.e. no histogram) you can get some idea of the range of values that Oracle generates through the hash function. For example:
Here’s a note which I drafted in Novemeber 2010, and then didn’t publish. I found it earlier on this morning while looking for another note I’d written about histograms so, even though it may not be something that people need so much these days, I thought: better late than never.
I’ve pointed out in the past that I’m not keen on seeing lots of histograms on a system and tend to delete them if I think they are not needed. Here’s an example of the type of code I use to delete a histogram.
In the past I have enthused mightily about the benefits of the approximate NDV mechanism and the benefit of using auto_sample_size to collect statistics in 11g; however, as so often happens with Oracle features, there’s a down-side or boundary condition, or edge case. I’ve already picked this up once as an addendum to an earlier blog note on virtual stats, which linked to an article on OTN describing how the time taken to collect stats on a table increased dramatically after the addition of an index – where the index had this definition:
Here’s another of my “draft” notes that needs some exapansion and, most importantly, proof.
I have a fact table with a “status id” column that shows a massive skew. Unfortunately I have a dimension table that holds the “status code”, so (in theory, at least) I have to do a join from the statuses table to the fact table to find rows of a given status. Unfortunately the join hides the skew:
select f.* from facts f, statuses s where s.code = 'C' and f.status_id = s.status_id ;
The optimizer knows that the status_id column on the facts table has a highly skewed distribution and will create a histogram on it, but it can’t know which status code corresponds to which status_id, so the histogram doesn’t help in calculating the join cardinality.
Will a bitmap join index help ? Answer – NO.
Last week, while working on customer engagement, I learned a new method of quantifying behavior of time-series data. The method is called “Control Chart” and credit to Josh Wills, our director of data science, for pointing it out. I thought I’ll share it with my readers as its easy to understand, easy to implement, flexible and very useful in many situations.
The problem is ages old – you collect measurements over time and want to know when your measurements indicate abnormal behavior. “Abnormal” is not well defined, and thats on purpose – we want our method to be flexible enough to match what you define as an issue.
For example, lets say Facebook are interested in tracking usage trend for each user, catching those with decreasing use
There are few steps to the Control Chart method:
It has taken much longer than I anticipated to get around to writing part 3 of this mini-series on what Oracle has done about histograms in 12c.
In part 1 I gave a thumbnail sketch of the three types of histogram available in 12c
In part 2 I described in some detail the improvements in performance and accuracy for the frequency and top-frequency histograms
Or – to be more accurate – real statistics on a virtual column.
This is one of the “10 top tips” that I came up with for my session with Maria Colgan at OOW13. A method of giving more information that might improve execution plans when you can change the code. I’ll start with a small data set including a virtual column (running 126.96.36.199), and a couple of problem queries:
Here’s a little demo cut-n-pasted from a session running Oracle 188.8.131.52 (it works on 11g, too). All it does is create a table by copying from a well-known table, gather extended stats on a column group, then show you the resulting column names by querying view user_tab_cols.