Over the past couple days I’ve been reading through the recent paper by Karen Morton entitled “Managing Statistics for Optimal Query Performance“. In this paper Karen goes over many of the topics I have discussed as well (and a few that I have not) in the following blog posts: Troubleshooting Bad Execution Plans There...
I’ve written before on choosing an optimal stats gathering strategy but I recently came across a scenario that I didn’t directly blog about and think it deserves attention. As I mentioned in that previous post, one should only deviate from the defaults when they have a reason to, and fully understand that reason and...
Note: Originally this experiment was from a post I wrote on the Oracle Forum: Database – General. I recommend that you read Jonathan Lewis’ summarization of the thread instead of reading all 671 posts (as of today). You will spend much less time and get more out of the discussion. One of the new...
Previously I blogged about the new and improved DBMS_STATS.AUTO_SAMPLE_SIZE used to calculate NDV in Oracle 11g and now I wanted to touch on another new feature of DBMS_STATS in 11g: Incremental Global Statistics On Partitioned Tables. Before Incremental Global Stats (Two-Pass Method) When DBMS_STATS.GATHER_TABLE_STATS collects statistics on a partitioned table, generally it does so...
Recently I was reading this thread, “Trying to make use of bitmap indexes” on the Oracle Forum. Before I had finished a working example, Jonathan Lewis had posted his response which was on par with my thoughts. Since this is a topic I see frequently, I thought I would finish my experiment and publish...
Recently the Oracle Optimizer Development Team put out a White Paper entitled Upgrading from Oracle Database 9i to 10g: What to expect from the Optimizer. This paper discusses the main differences between 9i and 10g in the subject area of the Optimizer and Statistics. As G.I. Joe said, “Now we know! And knowing is...
I recently came across a query in which the Optimizer was making a poor cardinality estimate, which in turn caused inefficient join type, which in turn caused the query to run excessively long. This post is a reenactment of my troubleshooting. The Suspect SQL The original SQL was quite large and had a fairly...
I’ve been working on a few test cases and I’m in search of some real-world data. If your production Oracle database uses system statistics, either Workload Statistics or Noworkload Statistics, and you are willing to share them, please post a comment with the output from the following two queries: select version from v$instance; select...
Since the introduction of the Cost Based Optimizer (CBO) in Oracle 7.0, people have been both fascinated and terrified by it and the statistics that feed it. There has long been a belief that a degree in witchcraft or black magic is required to successfully work with the CBO. Some people feel this shroud...
One of the most common performance issues DBAs encounter are bad execution plans. Many try to resolve bad executions plans by setting optimizer related parameters or even hidden underscore parameters. Some even try to decipher a long and complex 10053 trace in hopes to find an answer. While changing parameters or analyzing a 10053...
Oracle OpenWorld 2007 has come and gone and from the Real-World Performance Group’s perspective we’d consider it a successful one. The content of this year’s presentations seems to have gone over quite well as shown by the number of hallway comments and emails we received. At least two Oracle bloggers have put up their...
In the Real-World Performance Roundtable, Part 2: The Optimizer, Schema Statistics, SQL Tuning at Oracle OpenWorld 2006, I worked an example of how the optimizer can have difficulty estimating the correct cardinality when there is data correlation. (The Zodiac example can be found on pages 46-49 of the presentation.) In Oracle 11g, there has...