oracle: Is there a way to check what sql_id downgraded to serial or lesser degree over the period of time - multithreading

Question

I would like to know if there is a way to check sql_ids that were downgraded to either serial or lesser degree in an Oracle 4-node RAC Data warehouse, version 11.2.0.3. I want to write a script and check the queries that are downgraded.
SELECT NAME, inst_id, VALUE FROM GV$SYSSTAT
WHERE UPPER (NAME) LIKE '%PARALLEL OPERATIONS%'
OR UPPER (NAME) LIKE '%PARALLELIZED%' OR UPPER (NAME) LIKE '%PX%'
NAME VALUE
queries parallelized 56083
DML statements parallelized 6
DDL statements parallelized 160
DFO trees parallelized 56249
Parallel operations not downgraded 56128
Parallel operations downgraded to serial 951
Parallel operations downgraded 75 to 99 pct 0
Parallel operations downgraded 50 to 75 pct 0
Parallel operations downgraded 25 to 50 pct 119
Parallel operations downgraded 1 to 25 pct 2
Does it ever refresh? What conclusion can be drawn from above output? Is it for a day? month? hour? since startup?

Answer 1

This information is stored as part of Real-Time SQL Monitoring. But it requires licensing the Diagnostics and Tuning packs, and it only stores data for a short period of time.
Oracle 12c can supposedly store SQL Monitoring data for longer periods of time. If you don't have Oracle 12c, or if you don't have those options licensed, you'll need to create your own monitoring tool.
Real-Time SQL Monitoring of Parallel Downgrades
select /*+ parallel(1000) */ * from dba_objects;
select sql_id, sql_text, px_servers_requested, px_servers_allocated
from v$sql_monitor
where px_servers_requested <> px_servers_allocated;
SQL_ID SQL_TEXT PX_SERVERS_REQUESTED PX_SERVERS_ALLOCATED
6gtf8np006p9g select /*+ parallel ... 3000 64
Creating a (Simple) Historical Monitoring Tool
Simplicity is the key here. Real-Time SQL Monitoring is deceptively simple and you could easily spend weeks trying to recreate even a tiny portion of it. Keep in mind that you only need to sample a very small amount of all activity to get enough information to troubleshoot. For example, just store the results of GV$SESSION or GV$SQL_MONITOR (if you have the license) every minute. If the query doesn't show up from sampling every minute then it's not a performance issue and can be ignored.
For example: create a table create table downgrade_check(sql_id varchar2(100), total number), and create a job with DBMS_SCHEDULER to run insert into downgrade_check select sql_id, count(*) total from gv$session where sql_id is not null group by sql_id;. Although the count from GV$SESSION will rarely be exactly the same as the DOP.
Other Questions
V$SYSSTAT is updated pretty frequently (every few seconds?), and represents the total number of events since the instance started.
It's difficult to draw many conclusions from those numbers. From my experience, having only 2% of your statements downgraded is a good sign. You likely either have good (usually default) settings and not too many parallel jobs running at once.
However, some parallel queries run for seconds and some run for weeks. If the wrong job is downgraded even a single downgrade can be disastrous. Storing some historical session information (or using DBA_HIST_ACTIVE_SESSION_HISTORY) may help you find out if your critical jobs were affected.