A sql like "SELECT COUNT(1) FROM tablename..." was executed by my springboot application with mybatis,but no response during a long time.
But when I executed the same sql directly in ORACLE,it worked and returned the response.
With executing these sql:
select event,count(*) from v$session_wait group by event order by 2 desc;
I found the 'latch:cache buffers chains' event.
I asked my frined and he told me that I can execute analyze table tablename compute statistics.
After executed the sql,the problem resolved,the select count sql executed by springboot application worked !
I feel puzzled,why the analyze sql worked?
Thank you.
Related
I am new to oracle. I am using oracle database 19c. I need to restrict the query execution in such a way that if a query is executed once, concurrent execution should get warning or wait until the first query is complete.
Example:
--First time execution
select count(*) from table;
--At the same time, another user execute the query
select count(*) from table; <<== Either wait or get a warning
Thanks in advance !
I'm getting some database errors from an application related to unique constraints however none of the records have any duplicate values so I want to check what query was run that produced the error.
How can I check a history log of queries to pinpoint the problem? I'm using Oracle 12c
You can query the Oracle V$SQL view:
SELECT *
FROM v$sql s
WHERE PARSING_SCHEMA_NAME = 'FOO'
ORDER BY s.LAST_ACTIVE_TIME DESC nulls last;
In this example the result is already filtered by PARSING_SCHEMA_NAME which is the user name that executed the sql.
See Oracle Documentation on V$SQL for details
I'm building a Java EE web application using Servlets.
In a servlet I need to perform a series (30) of queries to an Oracle Database.
The queries are all of this type: count(*) from TABLE where CONDITIONS.
Each query takes 1 second, not too much, but the total amount of time is 30-35 seconds: that's too much!
Have you got any idea how I could improve the performances of my queries?
Is there a way to run them in parallel?
You could try to make one query of the 30:
select
(select count(*) from tbl_table1 where ...) as table1_cnt,
(select count(*) from tbl_table2 where ...) as table2_cnt,
...
(select count(*) from tbl_table30 where ...) as table30_cnt
from dual;
If this doesn't help and you cannot alter the db, one thing remained. Cache your query results at Java side, in the cache keep track of your insert/delete operations made by the Java application, and time-by-time refresh your cache from db in the background using a scheduler.
I have an application that executes some sql queries. How can I get execution plan for currently executing query from sqlplus or some other oracle client? I can amend oracle session that is used in application, if that is necessary.
I do not want to use explain plan and execute that query by hand, I need to get actual execution plan that is used for query.
You can run explain plain on historical queries from the SGA -examples
And listing B.
Example:
SELECT username, prev_sql_id
FROM v$session
WHERE username = 'RDEVALL' -- example user
SQL ID returned
RDEVALL a1d4z5ruduzrd
RDEVALL avmbutuknvb6j
RDEVALL 75g0tqd9x743y
RDEVALL 5fnkh6c8mqqt3
RDEVALL 75g0tqd9x743y
Pick query ID and use here:
SELECT *
FROM TABLE(DBMS_XPLAN.DISPLAY_CURSOR('a1d4z5ruduzrd')); -- replace with sql ID as needed
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.