Oracle statistics collection

We all know good old “analyze table” SQL statement to collect statistics of a table. With newer version of Oracle database release, dbms_stats package becomes preferred method and it is much more powerful than “Analyze table” SQL. Starting with Oracle 10g, oracle uses dedicated background process to collect database statistics and it is on by default. However, because oracle considering column cardinality is independent to each other, the optimizer can generate bad execution plan if the column’s data distributions are related, such as column state and column zip code. To make the oracle to do the right thing, we can use dynamic sampling hint to ask Oracle collect statistics on fly just for the query to be run to get correct cardinality. Dynamic sampling does add some load tax on database and Oracle would not store the good execution plan generated with dynamic sampling. Start with Oracle 10g, Oracle SQL profile would collect stats for a query and store the execution plan to run such query all the time later. It is particularly useful that you can modify the SQL profile manually to let a query run the execution plan you like and with “force match” option, the execution plan can be applied to similar query that does not use binding variables. Add hint to a SQL becomes so much a thing of yesterday with SQL profile.