Why does Oracle performance go bad after a migration?

So, what do you do if you find that your well-optimized database performs poorly after a migration to a new release?  In my experience it?s most likely sub-optimal SQL execution, and rarely, sub-optimal PL/SQL.  I follow these steps:

1 ? Compare an execution plan from before the migration to a post-migration execution plan

2 ? Ensure that you have re-analyzed your schema using the dbms_stats utility

3 ? Check the Oracle docs for recommended changes to init.ora parameters.

Here are details:

1 - Find a super bad-performing SQL and dive-in to see exactly why the SQL is executing in a sub-optimal fashion.  Use the "set autotrace traceonly explain" command in SQL<*Plus, get the execution plan, and compare the execution plan to the same SQL in your old database.  For details on evaluating execution plans, see this article:

   http://www.dba-oracle.com/art_orafaq_oracle_sql_tune_table_scan.htm

2 - Make sure to re-analyze your schema using dbms_stats.  High-quality metadata can make all the difference.  Read this:

    http://www.dba-oracle.com/art_otn_cbo_p2.htm 

3 - Try re-setting Oracle initialization parameters: 

http://www.dba-oracle.com/art_builder_sql_parm.htm

• optimizer_mode ? If you have an OLTP database, try first_rows.  Also, if your old optimizer_mode was ?choose? you will need to pick a new one in Oracle10g, usually all_rows or first_rows (sometimes first_rows_100 too).
   
• optimizer_index_cost_adj = 25 ? This adjusts the optimizer to favor index access
   
• optimizer_index_caching = set to percentage of indexes that are buffered according to x$bh buffer data.  A script to give you the best initial setting can be downloaded here:

             http://www.rampant-books.com/book_1002_oracle_tuning_definitive_reference_2nd_ed.htm

Remember, the all_rows optimizer mode is designed to minimize computing resources and it favors full-table scans.  Index access (first_rows) adds additional I/O overhead, but they return rows faster, back to the originating query:

Get the Complete Oracle SQL Tuning Information  The landmark book "Advanced Oracle SQL Tuning  The Definitive Reference"  is filled with valuable information on Oracle SQL Tuning. This book includes scripts and tools to hypercharge Oracle 11g performance and you can buy it for 30% off directly from the publisher.