Merge pull request oracle-samples#65 from nigelbayliss/master

Using SPM to repair regressed SQL execution plans

Author: nigelbayliss

optimizer/spm_fix/README.md

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+This demo shows you how you can use SQL plan management (SPM) to fix a SQL statement that is experiencing a performance regression caused by a sub-optimal plan.
+
+SPM will search for historic plans, choose the best one and enforce it with a SQL plan baseline.
+
+This demonstration is intended for use in Oracle Database 18c onwards. The primary script is "spm.sql", which demonstrates how to use SPM to find and implement a better plan.
+
+Create the user, create the tables and then run "example.sql". It works as follows:
+
+- Tables T1 and T2 have data skew
+- Q1 is a query that joins T1 and T2
+- Histograms tell the optimizer about the skew so Q1 performs well
+- We drop the histograms and this induces a poor plan for Q1
+- SPM is initiated and it finds the previous good plan
+- The good plan is tested (automatically) by SPM and a SQL plan baseline is created
+- Q1 now uses the good plan
+
+```
+$ sqlplus / as sysdba     [or connect to PDB ADMIN]
+SQL> @@user
+SQL> connect spmdemo/spmdemo
+-- 
+-- Create test tables
+-- 
+SQL> @@tab
+--
+-- Review/execute the following script
+--
+SQL> @@example
+```
+
+Note that AWR is accessed. Check the Oracle Database License Guide for details.
+
+The test creates two tables T1 and T2 - use a test database
+
+DISCLAIMER:
+   <br/>-- These scripts are provided for educational purposes only.
+   <br/>-- They are NOT supported by Oracle World Wide Technical Support.
+   <br/>-- The scripts have been tested and they appear to work as intended.
+   <br/>-- You should always run scripts on a test instance.
+
+

optimizer/spm_fix/drop.sql

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+DECLARE
+  l_plans_dropped  PLS_INTEGER;
+BEGIN
+
+  FOR REC IN (SELECT DISTINCT SQL_HANDLE FROM DBA_SQL_PLAN_BASELINES )
+--              WHERE CREATOR = 'SPMDEMO')
+  LOOP
+      L_PLANS_DROPPED := DBMS_SPM.DROP_SQL_PLAN_BASELINE (
+        sql_handle => rec.sql_handle,
+        PLAN_NAME  => NULL);
+  END LOOP;
+
+END;
+/
+

optimizer/spm_fix/droph.sql

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+set echo on
+exec dbms_stats.delete_column_stats(user,'t1','d',no_invalidate=>false,col_stat_type=>'HISTOGRAM');
+exec dbms_stats.delete_column_stats(user,'t2','d',no_invalidate=>false,col_stat_type=>'HISTOGRAM');