By Franck Pachot . This extends a previous post (Rolling Invalidate Window Exceeded) where, in summary, the ideas were: When you gather statistics, you want the new executions to take into account the new statistics, which means that the old execution plans (child cursors) should be invalidated You don’t want all child cursors to be immediately invalidated, to avoid an…
By Franck Pachot . This was initially posted to CERN Database blog on Wednesday, 12 September 2018 where it seems to be lost. Here is a copy thanks to web.archive.org Demo table I create a table for the demo. The CTAS gathers statistics (12c online statistics gathering) with one row and then I insert more rows: 10:33:56 SQL> create table…
By Franck Pachot . I have written a lot about SQL Plan Directives that appeared in 12c. They were used by default and, because of some side effects at the time of 12cR1 with legacy applications that were parsing too much, they have been disabled by default in 12cR2. Today, there are probably not used enough because of their bad…
By Franck Pachot . Here is a little script I use from time to time to look at V$MYSTAT values and displaying on one line a set of statistics with their delta value between two calls. The first script, _mystat_init.sql, initializes the variables. The second one displays the values, such as: SQL> @ _mystat_diff.sql db block changes redo size…
By Franck Pachot . In my UKOUG OracleScene article on 12c online statistics and GTT I mentioned the following: A final note about those 12c changes in statistics gathering on GTT. In 11g the dbms_stats did a commit at the start. So if you did gather stats after the load, you had to set the GTT as ON COMMIT PRESERVE…
The Optimizer Statistics Advisor is a new Advisor in Oracle 12.2. The goal of this Advisor is to check the way you gather the statistics on your database, and depending on what is found, it will makes some recommendations on how you can improve the statistics gathering strategy in order to provide more efficient statistics to the CBO. This Advisor…
I would like to share with you an interesting issue you may face while using SQL Server AlwaysOn availability groups and secondary read-only replicas. For those who use secondary read-only replicas as reporting servers, keep reading this blog post because it is about update statistics behavior on the secondary replicas and as you may know cardinality estimation accuracy is an…
By Franck Pachot . Today I was doing a hard parse storm post-mortem analysis. One hypothesis was rolling invalidation causing invalidation, but figures didn’t match. I often reproduce the hypothesis to check the numbers to be sure I interpret them correctly. Especially the timestamps in V$SQL_SHARED_CURSOR.REASON. And as it may help others (including myself in the future) I share the…
Sometimes, you need to modify the statistics of a table. There are various reasons: either for the performance analysis or because the execution plan changed. Oracle offers two methods: statistics restore or statistics export. Both are in the dbms_stats package. 1. Statistics restore This method uses the procedure dbms_stats.restore_%_stats, where % can be table, schema, database, and fixed_objects. This is…
Nowadays MySQL Databases are encapsulated into many mission critical software solutions. Lots of companies host one or many MySQL databases in their data center, sometimes even without knowing it except when the MySQL service is no more available. In order to increase this service availability it is mandatory to have a monitoring solution. Regardles of you are using MySQL Server,
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