I wrote an application that queries oracle v$sqlarea and dumps data to my own database for further analysis. I noticed something very strange - sometimes data in the v$sqlarea shows less executions than before. I'm pretty sure that the oracle cache was not cleaned (the first load time of query is still the same, and since I query oracle each minute I dont believe that in this one minute the query was executed 100k+ times).
Can anybody explain how this is possible?
Ok, so I asked this on Oracle forum as well, and I believe that the correct answer is this one
https://community.oracle.com/message/12980175#12980175
Related
Being production support team member, I investigate issues with various Impala queries and while researching on an issue , I see a team submits an Impala query with LIMIT 0 which obviously do not return any rows and then again without LIMIT 0 which gives them result. I guess they submit these queries from IBM Datastage. Before I question them why they do so.. wanted to check what could be a reason for someone to run with LIMIT 0. Is it just to check syntax or connection with Impala? I see a similar question discussed here in context of SQL but thought to ask anyway in Impala perspective. Thanks Neel
I think you are partially correct.
Pls note, limit will process all the data and then apply limit clause.
LIMIT 0 is mostly used to -
to check if syntax of SQL is correct. But impala do fetch all the records before applying limit. so SQL is completely validated. Some system may use this to check out the sql they generated automatically before actually applying it in server.
limit fetching lots of rows from a huge table or a data set every time you run a SQL.
sometime you want to create an empty table using structure of some other tables but do not want to copy store format, configurations etc.
dont want to burden the hue/any interface that is interacting with impala. All data will be processed but will not be returned.
performance test - this will somewhat give you an idea of run time of SQL. i used the word somewhat because its not actual time to complete but estimated time to complete a SQL.
I have a SQL query that fetches roughly 200 columns from multiple tables and normally runs in a matter of minutes.
A Java program kicked off by cron calls the SQL every 4 hours, but occasionally hangs forever(=not fetching any data. Neither updates nor inserts are involved).
Here are some outputs from V$SESSION.
STATUS: ACTIVE
ROW_WAIT_OBJ#: 22392 ←not changing
ROW_WAIT_FILE#: 6 ←not changing
ROW_WAIT_BLOCK#: 8896642 ←not changing
ROW_WAIT_ROW#: 0 ←not changing
LAST_CALL_ET: 5632 ←keeps incresing
★No other heavy SQL queries are running at the same time
What could be the cause of this and what should I look into to solve it?
You can use TKPROF or SQL Profiler. This reports can help you. We can not replay your question now.
If you attach your tuning reports, we can help you. Because many things can cause performance problems. A comprehensive study is needed to understand this.
Follow this link;
https://docs.oracle.com/cd/E11882_01/server.112/e41573/perf_overview.htm
I run a complex query against Oracle DB 11G based eBS R12 schema:
For first run it takes 4 seconds. If I run it again, it takes 9, next 30 etc.
If I add "and 1=1" it takes 4 seconds again, then 9, the 30 and so on.
Quick workaraound is that we added a random generated "and sometstring = somestring" and now the results are always in 4 second.
I have never encoutered a query that would behave this way (it should be opposite, or no siginificat change between executions). We tested it on 2 copies of same DB, same behaviour.
How to debug it? And what internal mechanics could be getting confused?
UPDATE 1:
EXPLAIN PLAN FOR
(my query);
SELECT * FROM table(DBMS_XPLAN.DISPLAY);
Is exactly the same before first run that it is for subsequent ones. see http://pastebin.com/dMsXmhtG
Check the DBMS_XPLAN.DISPLAY_CURSOR. The reason could be cardinality feedback or other adaptive techniques Oracle uses. You should see multiple child cursors related to SQL_ID of your query and you can compare their plans.
Has your query bound variables and columns used for filtering histograms? This could be another reason.
Sounds like you might be suffering from adaptive cursor sharing or cardinality feedback. Here is an article showing how to turn them off - perhaps you could do that and see if the issue stops happening, as well as using #OldProgrammer's suggestion of tracing what is happening.
If one of these is found to be the problem, you can then take the necessary steps to ensure that the root cause (eg. incorrect statistics, unnecessary histograms, etc.) is corrected.
Not sure if this question has been already asked. I face this problem where the 1st hit from the website to an Oracle SP takes a lot of time. Subsequent accesses work just fine.
The SP i'm taking about here is a dynamic SP used for Search functionality(With different search criteria selection option available)
1st access time ~200 seconds
subsequent access time ~20 to 30 seconds.
Stored Procedure logic on a high level.
Conditional JOINS are appended based on some logics.
Dynamic SQL and cursor used to retrieve data.
Any help to start tackling these kind of issues is very helpful..
Thanks,
Adarsh
The reason why it takes only a few secs to execute the query after the first run is that Oracle caches the results. If you change the SQL then Oracle considers it a different query and won't serve the results from the cache but executes the new query (even formatting the code again or adding a space in between will be a difference).
It is a hard question how to speed up first execution. You'll need to post your query and explain plan and probably you'll have to answer further questions if you want to get help on that.
I am trying to understand the performance of a query that I've written in Oracle. At this time I only have access to SQLDeveloper and its execution timer. I can run SHOW PLAN but cannot use the auto trace function.
The query that I've written runs in about 1.8 seconds when I press "execute query" (F9) in SQLDeveloper. I know that this is only fetching the first fifty rows by default, but can I at least be certain that the 1.8 seconds encompasses the total execution time plus the time to deliver the first 50 rows to my client?
When I wrap this query in a stored procedure (returning the results via an OUT REF CURSOR) and try to use it from an external application (SQL Server Reporting Services), the query takes over one minute to run. I get similar performance when I press "run script" (F5) in SQLDeveloper. It seems that the difference here is that in these two scenarios, Oracle has to transmit all of the rows back rather than the first 50. This leads me to believe that there is some network connectivity issues between the client PC and Oracle instance.
My query only returns about 8000 rows so this performance is surprising. To try to prove my theory above about the latency, I ran some code like this in SQLDeveloper:
declare
tmp sys_refcursor;
begin
my_proc(null, null, null, tmp);
end;
...And this runs in about two seconds. Again, does SQLDeveloper's execution clock accurately indicate the execution time of the query? Or am I missing something and is it possible that it is in fact my query which needs tuning?
Can anybody please offer me any insight on this based on the limited tools I have available? Or should I try to involve the DBA to do some further analysis?
"I know that this is only fetching the
first fifty rows by default, but can I
at least be certain that the 1.8
seconds encompasses the total
execution time plus the time to
deliver the first 50 rows to my
client?"
No, it is the time to return the first 50 rows. It doesn't necessarily require that the database has determined the entire result set.
Think about the table as an encyclopedia. If you want a list of animals with names beginning with 'A' or 'Z', you'll probably get Aardvarks and Alligators pretty quickly. It will take much longer to get Zebras as you'd have to read the entire book. If your query is doing a full table scan, it won't complete until it has read the entire table (or book), even if there is nothing to be picked up in anything after the first chapter (because it doesn't know there isn't anything important in there until it has read it).
declare
tmp sys_refcursor;
begin
my_proc(null, null, null, tmp);
end;
This piece of code does nothing. More specifically, it will parse the query to determine that the necessary tables, columns and privileges are in place. It will not actually execute the query or determine whether any rows meet the filter criteria.
If the query only returns 8000 rows it is unlikely that the network is a significant problem (unless they are very big rows).
Ask your DBA for a quick tutorial in performance tuning.