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
Related
So I am new in SQL DEVELOPER tools and I have written a simple select statement like:
SELECT * FROM employee;
it worked fine but there was a yellow warning mark underneath SELECT and I clicked on that and my query changes into the following query:
SELECT "A1"."EMPLOYEE_ID" "EMPLOYEE_ID","A1"."FIRST_NAME" "FIRST_NAME","A1"."LAST_NAME" "LAST_NAME","A1"."SALARY" "SALARY", "A1"."DEPARTMENT_ID" "DEPARTMENT_ID","A1"."MANAGER_ID" "MANAGER_ID","A1"."HIRE_DATE" "HIRE_DATE"
FROM "INTRO_USER"."EMPLOYEE" "A1";
My Quest is what is the difference between these two queries? although their output is the same
The glob * has been expanded to all column of the table. The table name EMPLOYEE is aliased to A1 to make it shorter.
The feature you are seeing is called 'SQL Text Expansion,' and it's setup to allow you to see what your query would look like if you were working with one or more VIEWS.
For example, SELECT * FROM ALL_TABLES is quite complicated. This feature allows you to see what's actually involved when running that SQL.
https://www.thatjeffsmith.com/archive/2014/12/sql-developer-and-a-12c-magic-trick/
There is probably no change or expected delta in the performance or execution plan of the 2 versions of your query.
I created a table in oracle like
CREATE TABLE suppliers AS (SELECT * FROM companies WHERE id > 1000);
I would like to know the complete select statement which was used to create this table.
I have already tried get_ddl but it is not giving the select statement. Can you please let me know how to get the select statement?
If you're lucky one of these statements will show the DDL used to generate the table:
select *
from gv$sql
where lower(sql_fulltext) like '%create table suppliers%';
select *
from dba_hist_sqltext
where lower(sql_text) like '%create table%';
I used the word lucky because GV$SQL will usually only have results for a few hours or days, until the data is purged from the shared pool. DBA_HIST_SQLTEXT will only help if you have AWR enabled, the statement was run in the last X days that AWR is configured to hold data (the default is 8), the statement was run after the last snapshot collection (by default it happens every hour), and the statement ran long enough for AWR to think it's worth saving.
And for each table Oracle does not always store the full SQL. For security reasons, DDL statements are often truncated in the data dictionary. Don't be surprised if the text suddenly cuts off after the first N characters.
And depending on how the SQL is called the case and space may be different. Use lower and lots of wildcards to increase the chance of finding the statement.
TRY THIS:
select distinct table_name
from
all_tab_columns where column_name in
(
select column_name from
all_tab_columns
where table_name ='SUPPLIERS'
)
you can find table which created from table
I need to recreate some SAP stored procedures in Oracle. I've been trying to find tutorials, similar questions, examples, etc about this but apparently no one had to do this before
What Oracle SQL query can be similar to this SAP query ?
SELECT * FROM A
INTO CORRESPONDING FIELDS OF TABLE B
FOR ALL ENTRIES IN C
WHERE a = C-a
AND x = y.
LOOP AT B INTO D.
D-b = E-b.
INSERT c FROM D.
IF SY-SUBRC <> 0.
WRITE: / 'error on insert', D-b, D-a.
ENDIF.
Any help will be appreciated, Thanks.
I recommend you to use transaction 'ST05' to trace your program. This tool will show details of the queries on the database including the exact SQL executed.
EDIT:
As a demonstration of the queries generated by SAP for Oracle let's execute this code and trace it with transaction 'ST05'. Remember to run 'ST05' before executing the program.
tables: mara.
data: it_mara type standard table of mara,
it_eina type standard table of eina.
select-options so_matnr for mara-matnr.
start-of-selection.
select matnr from mara into corresponding fields of table it_mara
up to 100 rows where matnr in so_matnr.
check sy-subrc eq 0.
select * from eina into table it_eina for all entries in it_mara
where matnr eq it_mara-matnr.
After execution check the output in transaction 'ST05':
If you want more details select an SQL statement in the screen and then click the button 'Explain'. You will see the following:
For better reference on transaction 'ST05' check this link.
Hope it helps.
The FOR ALL ENTRIES statement usually produces many queries which results are then grouped by UNION or UNION ALL.
Here is a really nice analysis for Microsoft SQL Server.
Because of the fact that UNION and UNION ALL are part of SQL standard I think it is implemented exactly the same for any other SQL database.
[EDIT]
As Mr Miranda stated it looks differently when it comes to Oracle database. I googled a bit and found this article where it is said that IN-LISTs are used which seems also to be plausible.
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.
well this problem is general in sql server ce
i have indexes on all the the fields.
also the same query but with ID IN ( list of int ids) is pretty fast.
i tried to change the query to OUTER Join but this just make it worse.
so any hints on why this happen and how to fix this problem?
That's because the index is not really helpful for that kind of query, so the database has to do a full table scan. If the query is (for some reason) slower than a simple "SELECT * FROM TABLE", do that instead and filter the unwanted IDs in the program.
EDIT: by your comment, I recognize you use a subquery instead of a list. Because of that, there are three possible ways to do the same (hopefully one of them is faster):
Original statement:
select * from mytable where id not in (select id from othertable);
Alternative 1:
select * from mytable where not exists
(select 1 from othertable where mytable.id=othertable.id);
Alternative 2:
select * from mytable
minus
select mytable.* from mytable in join othertable on mytable.id=othertable.id;
Alternative 3: (ugly and hard to understand, but if everything else fails...)
select * from mytable
left outer join othertable on (mytable.id=othertable.id)
where othertable.id is null;
This is not a problem in SQL Server CE, but overall database.
The OPERATION IN is sargable and NOT IN is nonsargable.
What this mean ?
Search ARGument Able, thies mean that DBMS engine can take advantage of using index, for Non Search ARGument Ablee the index can't be used.
The solution might be using filter statement to remove those IDs
More in SQL Performance Tuning by Peter Gulutzan.
ammoQ is right, index does not help much with your query. Depending on distribution of values in your ID column you could optimise the query by specifying which IDs to select rather than not to select. If you end up requesting say more than ~25% of the table index will not be used anyway though because for nonclustered indexed (which is the only type of indexes which SQL CE supports if memory serves) it would be cheaper to scan the table. Otherwise (if the query is actually selective) you could re-write query with ID ranges to select ('union all' may work better than 'or' to combine ranges if SQL CE supports 'union all', not sure)