I'm trying to improve performance to specific query in my DB.
One of the tables in the query, has parallel degree which is greater than 1 in dba_tables.
I founded that if I add a hint of
NO_PARALLEL
my query back to run very fast.
The problem is that I don't want to touch and change the source of query.
So I found this article:
http://intermediatesql.com/oracle/how-to-add-a-hint-to-oracle-query-without-touching-its-text/
which explain how to use alias with block names to create a profile and actually execute the hint with no touching the query.
I used it before for use an index even the optimizer decide to execute FTS.
But in case of parallel - I just didn't understand how to do that.
I just want to add the action that will simulate the no_parallel hint on the whole query.
Additionally, I don't want to set the parallel parameter or alter the session.. just change for this query.
Someone?
Thanks.
Forcing a single query to use /*+ NO_PARALLEL */ is easy because it's a statement-level hint. There's no need to add query block names or figure out Oracle's internal names.
Add the NO_PARALLEL hint to the query:
begin
dbms_sqltune.import_sql_profile(
sql_text => 'select /*+ parallel */ * from dba_objects',
profile => sqlprof_attr('no_parallel'),
name => '0ftu9j02g81b0_noparallel'
);
end;
/
Verify that the statement has a profile and does not run in parallel:
explain plan for select /*+ parallel */ * from dba_objects;
select * from table(dbms_xplan.display);
Results:
...
Note
-----
- Degree of Parallelism is 1 because of hint
- SQL profile "0ftu9j02g81b0_noparallel" used for this statement
- this is an adaptive plan
So you do not want to:
change the query itself
change the degree on the table
change session level parameters
It looks like a competency problem that a technical one.
If your query has fixed knows sql_id you can still use sqlpatch to force the hint during parsing. sqlplatch is not the same as profile, and maybe it more suits your needs.
Related
One of the job schedulers is running in the production environment on a daily basis which use to take only 20 mins based past execution history, but today it's been more than 2 hours still not completed.
a) How to check whether the SQL plan has changed today or not?
b) What could be the reasons for the plan change? One I know due to code change. What else could cause plan change?
You can check if the SQL execution plan has changed by using the Active Workload Repository (AWR). First, you need to find the SQL_ID for the relevant query. The view GV$SQL contains the most recent SQL. If you can't find the query in this view, try DBA_HIST_SQLTEXT instead.
select sql_id, sql_text
from gv$sql
where lower(sql_fulltext) like '%some unique string%';
With the SQL_ID, you can start investigating historical information. The table DBA_HIST_SQLSTAT contains lots of summary information about the SQL. The most important column is PLAN_HASH_VALUE; if that value changes, then the execution plan has changed.
select snap_id, sql_id, plan_hash_value, executions_delta, elapsed_time_delta/100000 seconds_delta
,dba_hist_sqlstat.*
from dba_hist_sqlstat
--join to dba_hist_snapshot if you want to find precise times instead of SNAP_IDs.
where sql_id = '&SQL_ID'
order by dba_hist_sqlstat.snap_id;
If the plan has changed, you can view both plans with this:
select * from table(dbms_xplan.display_awr(sql_id => '&SQL_ID'));
Unfortunately, the most difficult part of query tuning with Oracle is that there are a dozen different ways to view the execution plans, and each of them provides slightly different data.
This query only returns numbers for the last execution, but it returns actual numbers and times, which helps you focus on the specific operation and wait events that caused the problem.
select dbms_sqltune.report_sql_monitor(sql_id => '&SQL_ID', type => 'text') from dual;
This query returns some additional execution plan information, specifically the Note section. Most graphical IDEs leave out that section, but it's vital for complex troubleshooting. If something weird is going on, the Note section will often explain why.
select * from table(dbms_xplan.display_cursor(sql_id => '&SQL_ID'));
There are many reasons why execution plans can change. If you add additional information to the question I may be able to make an educated guess.
Quick Check :
Please check whether the Statistics, is upto Date, both System and Table statistics.
Pleae check if any changes to table or index made ?
I have a performance problem.
First PL/SQL (most time never ends and OS database process is always over 90%):
DECLARE
myId nvarchar2(10) := '0;WF21izb0';
BEGIN
insert into MY_TABLE (select * from MY_VIEW where ID = myId);
END;
Second PL/SQL (ends with successfull result in 50s):
BEGIN
insert into MY_TABLE (select * from MY_VIEW where ID = '0;WF21izb0');
END;
select count(*) from MY_VIEW
is also a not ending call, there are a lot of table joins behind this view.
select count(*) from MY_VIEW where ID = '0;WF21izb0'
ends in 50s with count=60000.
Can somebody explain me the reason why my first PL/SQL is not finishing after 50s? What is the difference between using static string and declared parameter?
It boils down to what the DB engine knows about your data and your query, when preparing the query execution plan.
When a literal is placed in your query, it is a part of your query, so is known to the engine responsible for preparing the plan. It can take that literal value into account and decide on an execution plan, that is suitable, e.g. based on the DB data statistics (e.g. that this value is rare).
When you are using a PL/SQL variable, the actual query, for which the plan is determined, is different. It's something like:
insert into MY_TABLE (select * from MY_VIEW where ID = :param)
As you can see, the DB engine has now no information on the value, which will be used, when the query gets executed. So the best plan for such a scenario, is to prepare something, which is averagely good for most of the probable values (i.e. see what values in the DB will match this place most often, i.e. values that are prevalent).
If your data is unbalanced, and the '0;WF21izb0' value is rare (or even non-existent) in your data, a selective index may be used to narrow down, what needs to be processed, relatively soon in the critical parts of the execution plan. This plan will however backfire, when you'll use a value, which is all over the place - use of the index will be counter-productive. A better plan for such case may be a full table scan. Possibly the same one, which is used when executing select count(*) from MY_VIEW.
If you are faced with a scenario, where you do not know the filtering value upfront, you'll have to analyze the view code, and try to adjust it so it can be effectively used also for less "selective" values. You could try applying some optimizer hints to the query. You could also resign from using a view, and try your luck with a tabular function, where you can push your filtering predicates to the exact spots of the query, where they can be used most effectively.
Edit:
All in all, follow the advises from the question comments, and examine your execution plans and execution profile data. You should be able to find the culprit. From there it may not be obvious, what the solution is, but still, you know your data and relations much better than us.
I was checking some traces, but after reading the comment of APC and the answer of Hilarion i end up in this solution:
declare
sql_stmt VARCHAR2(200);
id VARCHAR2(10) := '0;WF21izb0';
BEGIN
sql_stmt := 'insert into MY_TABLE (select * from MY_VIEW where ID = :1)';
EXECUTE IMMEDIATE sql_stmt using id;
END;
This is done in 50s, and id can be now a function/procedure parameter.
Thanks for the comments.
I'm learning about database indexes right now, and I'm trying to understand the efficiency of using them.
I'd like to see whether a specific query uses an index.
I want to actually see the difference between executing the query using an index and without using the index (so I want to see the execution plan for my query).
I am using sql+.
How do I see the execution plan and where can I found in it the information telling me whether my index was used or not?
Try using this code to first explain and then see the plan:
Explain the plan:
explain plan
for
select * from table_name where ...;
See the plan:
select * from table(dbms_xplan.display);
Edit: Removed the brackets
The estimated SQL execution plan
The estimated execution plan is generated by the Optimizer without executing the SQL query. You can generate the estimated execution plan from any SQL client using EXPLAIN PLAN FOR or you can use Oracle SQL Developer for this task.
EXPLAIN PLAN FOR
When using Oracle, if you prepend the EXPLAIN PLAN FOR command to a given SQL query, the database will store the estimated execution plan in the associated PLAN_TABLE:
EXPLAIN PLAN FOR
SELECT p.id
FROM post p
WHERE EXISTS (
SELECT 1
FROM post_comment pc
WHERE
pc.post_id = p.id AND
pc.review = 'Bingo'
)
ORDER BY p.title
OFFSET 20 ROWS
FETCH NEXT 10 ROWS ONLY
To view the estimated execution plan, you need to use DBMS_XPLAN.DISPLAY, as illustrated in the following example:
SELECT *
FROM TABLE(DBMS_XPLAN.DISPLAY (FORMAT=>'ALL +OUTLINE'))
The ALL +OUTLINE formatting option allows you to get more details about the estimated execution plan than using the default formatting option.
Oracle SQL Developer
If you have installed SQL Developer, you can easily get the estimated execution plan for any SQL query without having to prepend the EXPLAIN PLAN FOR command:
##The actual SQL execution plan
The actual SQL execution plan is generated by the Optimizer when running the SQL query. So, unlike the estimated Execution Plan, you need to execute the SQL query in order to get its actual execution plan.
The actual plan should not differ significantly from the estimated one, as long as the table statistics have been properly collected by the underlying relational database.
GATHER_PLAN_STATISTICS query hint
To instruct Oracle to store the actual execution plan for a given SQL query, you can use the GATHER_PLAN_STATISTICS query hint:
SELECT /*+ GATHER_PLAN_STATISTICS */
p.id
FROM post p
WHERE EXISTS (
SELECT 1
FROM post_comment pc
WHERE
pc.post_id = p.id AND
pc.review = 'Bingo'
)
ORDER BY p.title
OFFSET 20 ROWS
FETCH NEXT 10 ROWS ONLY
To visualize the actual execution plan, you can use DBMS_XPLAN.DISPLAY_CURSOR:
SELECT *
FROM TABLE(DBMS_XPLAN.DISPLAY_CURSOR(FORMAT=>'ALLSTATS LAST ALL +OUTLINE'))
Enable STATISTICS for all queries
If you want to get the execution plans for all queries generated within a given session, you can set the STATISTICS_LEVEL session configuration to ALL:
ALTER SESSION SET STATISTICS_LEVEL='ALL'
This will have the same effect as setting the GATHER_PLAN_STATISTICS query hint on every execution query. So, just like with the GATHER_PLAN_STATISTICS query hint, you can use DBMS_XPLAN.DISPLAY_CURSOR to view the actual execution plan.
You should reset the STATISTICS_LEVEL setting to the default mode once you are done collecting the execution plans you were interested in. This is very important, especially if you are using connection pooling, and database connections get reused.
ALTER SESSION SET STATISTICS_LEVEL='TYPICAL'
Take a look at Explain Plan. EXPLAIN works across many db types.
For sqlPlus specifically, see sqlplus's AUTO TRACE facility.
Try this:
http://www.dba-oracle.com/t_explain_plan.htm
The execution plan will mention the index whenever it is used. Just read through the execution plan.
such as:
select country
from table1
inner join table2 on table1.id=table2.id
where table1.name='a' and table2.name='b'
group by country
after the parse, which part will be executed first?
It looks like you want to know the execution plan chosen by Oracle. You can get that ouput from Oracle itself:
set serveroutput off
< your query with hint "/*+ gather_plan_statistics */" inserted after SELECT >
select * from table(dbms_xplan.display_cursor(null, null, 'last allstats'));
See here for an explanation how to read a query plan: http://download.oracle.com/docs/cd/E11882_01/server.112/e16638/ex_plan.htm#i16971
Be aware however that the choice of a query plan is not fixed. Oracle tries to find the currently best query plan, based on available statistics data.
There are plenty of places you can find the order in which SQL is executed:
FROM clause
WHERE clause
GROUP BY clause
HAVING clause
SELECT clause
ORDER BY clause
But note that this is the "theoretical" order - SQL engines are allowed to perform the operations in other orders, provided that the end result appears to have been produced by using the above order.
If you install the free tool SQL*Developer from Oracle, then you can click a button to get the explain plan.
A quick explanation is at http://www.seeingwithc.org/sqltuning.html
I have created a table with parallelism degree 2, however it doesn't seem like it's actually working.
Post your SQL. You can add a parallel hint, something like:
select /*+ parallel(d, 2) */
from someTable d
where blah;
Note that you need to use any aliases you used in the query (d in this case). Typically the driving table of the query is used here. Use explain plan to see if it changes after adding this hint.
its appears the i should get a full licensed version of oralcle