Re: Histogram Helper

Steve,

I too experienced dramatic improvement when using a couple of histograms on very heavily skewed columns.

The main thing with histograms is that you need to keep them updated. They do more harm than good if they get stale.

I experimented with creating histograms on all indexed columns. Didn't hurt anything but didn't help anything unless the data was skewed. I read somewhere (can't remember where) that if you can afford the processing time and the space to maintain the histograms, theoretically it shouldn't hurt to create them where they are not needed. I guess that's if you can't afford the time to figure out where the data is skewed or if you don't know how.

However, that seemed like a waste to me so we basically just use them sparingly where they can really help.

I've posted several times in the past about histograms and recieved very few replies. That leads me to believe that not many people are using or are interested in histograms. I don't see them heavily emphasized at conferences or in books.

Cherie Machler
Gelco Information Network

                                                                                       
                        
                    "Steve Orr"                                                        
                        
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I just experienced an incredibly DRAMATIC performance boost with a well-placed histogram! Now I'm hungry for "Histogram Helper." I'd like to analyze the database and identify other possible candidates for histograms.
(Then maybe review the code after that. I'm using CHOOSE optimization with
automatic statistics gathering on all tables and indexes.)

Here's a quote from the Oracle Tuning Guide: "In general, you should create histograms on columns that are frequently used in WHERE clauses of queries and have a highly skewed data distribution.
For many applications, it is appropriate to create histograms for all indexed columns because indexed columns typically are the columns most often
used in WHERE clauses."

Has anyone ever created histograms for all indexed columns as suggested above? You could query DBA_INDEXES (or DBA_IND_COLUMNS) and create dynamic SQL as follows: analyze table <TBL_NAME> compute statistics for all indexed columns size 10;

Better yet, you could set the "method_opt" parameter of the DBMS_STATS.GATHER_SCHEMA_STATS procedure to "all indexed columns size 100." But isn't that brute force? I'm thinking our histogram helper should be a little more sophisticated by factoring in an analysis of how skewed the data
is using the num_distinct, density, num_buckets, and last_analyzed columns from dba_tab_columns. Finally, histogram helper should be automated to ensure our statistics are maintained.

Any ideas or personal experiences with histograms? Any white papers on implementing and managing histograms?

TIA!
Steve Orr

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