I am trying to validate the usage of an index but I cannot find a command allowing to show the current execution plan?
SELECT /*+ FORCE_INDEX(Persons idx_income) */
* from Persons
WHERE income > 10000000 and age < 40;
Is there a command to do this ? How to validate ?
If you are using the shell utility, you can use the command show query that displays the query plan for a query.
Related
I have to study a table on monetdb that probably has many columns.
When I do
SELECT * from cat.data limit 1;
I get
1 tuple !5600 columns dropped!
Which I interpret as not getting all the columns from the console.
I am using mclient to connect to the database.
I tried withe DESC, DESCRIBE - didnt work. Any help?
Indeed. See mclient --help
It is possible to extend the width of your output to see more columns.
Alternatively, within the mclient console use the \d tablename command
You need to use \w-1 command before executing your query.
sql>\w-1
sql>SELECT * from cat.data limit 1 ;
This will show all the columns in the terminal. The text will be wrapped.
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.
I want to run Hive and Impala Explain and compute stats command from java code. So that I can use the collected information for my analysis purpose. If any one have any idea please help
You can run it as any other jdbc query against impala.
The compute stats query for a table called temp would be "compute stats temp" and you can pass this as an argument for the jdbc statement.execute
Similarly, to explain a query, say "select count( * ) from temp" the query to pass as an argument for statement.execute is "explain select count(*) from temp".
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