plsql_optimize_level and plsql_code_type parameters

Purpose
We are able to use plsql_optimize_level parameter for getting better execution performance on plsql codes. On the other way, plsql_code_type parameter is using to change compilation mode.

Usage
Check current parameter:
SQL> show parameter plsql

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
plsql_ccflags                        string
plsql_code_type                      string      INTERPRETED
plsql_debug                          boolean     FALSE
plsql_optimize_level                 integer     2
plsql_v2_compatibility               boolean     FALSE
plsql_warnings                       string      DISABLE:ALL

Apply the setting:
init.ora file:
*.plsql_optimize_level=3
*.plsql_code_type='NATIVE'

SQL> shutdown immediate;

SQL> startup upgrade;

SQL> @$ORACLE_HOME/rdbms/admin/dbmsupgnv.sql;

SQL> shutdown;

SQL> startup;

SQL> ALTER SYSTEM ENABLE RESTRICTED SESSION;

SQL> @$ORACLE_HOME/rdbms/admin/utlrp.sql;

SQL> ALTER SYSTEM DISABLE RESTRICTED SESSION;

Check lastly:
SQL> show parameter plsql

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
plsql_ccflags                        string
plsql_code_type                      string      NATIVE
plsql_debug                          boolean     FALSE
plsql_optimize_level                 integer     3
plsql_v2_compatibility               boolean     FALSE
plsql_warnings                       string      DISABLE:ALL

To see current status of objects;
SQL> 
SELECT TYPE, PLSQL_CODE_TYPE, COUNT(*) FROM DBA_PLSQL_OBJECT_SETTINGS
WHERE PLSQL_CODE_TYPE IS NOT NULL
GROUP BY TYPE, PLSQL_CODE_TYPE
ORDER BY TYPE, PLSQL_CODE_TYPE;

Deep notes
INTERPRETED -> PL/SQL library units compile as bytecode format
NATIVE -> PL/SQL library units compile as native (machine) code format

Ref: https://becomeadba.com/?pages-list

How to generate sql explain plan?

e.g.
prepare example table:
SQL>
create table ep_employees as select * from employees

observe costs values:
SQL>
explain plan for select * from ep_employees where job_id = 'SA_MAN';
-- Explained.

SQL> 
select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------
-------------------------
Plan hash value: 745119821

----------------------------------------------------------------------------------
| Id  | Operation         | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |              |     5 |   675 |     3   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| EP_EMPLOYEES |     5 |   675 |     3   (0)| 00:00:01 |
----------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter("JOB_ID"='SA_MAN')

Note
-----

   - dynamic sampling used for this statement (level=2)

Fix full access issue on ep_employees table:

SQL>
create index ep_emp_job_ix on ep_employees (job_id);

SQL> 
select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------
-------------------------
Plan hash value: 3994861717

---------------------------------------------------------------------------------------------
| Id  | Operation                   | Name          | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |               |     5 |   675 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| EP_EMPLOYEES  |     5 |   675 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | EP_EMP_JOB_IX |     1 |       |     1   (0)| 00:00:01 |
---------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("JOB_ID"='SA_MAN')

Note
-----

   - dynamic sampling used for this statement (level=2)

Results:
Costs are reduced so sql query performance got increased.

How to see TOP 10 SQL that consuming system resources highly?

e.g.
select * from
(
select * from v$sql
order by DISK_READS desc
)
where rownum < 11

options for order by:
RUNTIME_MEM, EXECUTIONS, DISK_READS, SORTS, ELAPSED_TIME

for old querries:
select * from dba_hist_sqltext where sql_id = '8j2hv3c6wskdy'

for SQL plan:
select * from V$SQL_PLAN where sql_id = '8j2hv3c6wskdy'

find the object location of SQL
select * from all_source where owner <> 'SYS' and upper(text) like '%ABC%'