Is it only the explain plan to look in when tuning a large sql string?
Because when I push Ctrl+E in TOAD for Oracle (which generates explain plan), this takes several seconds. Does TOAD do anything more than generating the explain plan, or is it really so that the parse phase takes 2-3 seconds for that specific sql statement?
I really can't see how to optimize the sql string anymore when looking at the explain plan. So I thought maybe there is something going on BEFORE executing the plan?
thanks in advance
Martin (newbie oracle tuning expert)
The explain plan doesn't tell you everything - Oracle just tells you an estimate of the costs of your query.
To get the real costs of your query, you have to actually execute it and check the performance afterwards (e.g. using tkprof).
I'd recommend checking out Asktom, e.g.
http://asktom.oracle.com/pls/apex/f?p=100:11:0::::P11_QUESTION_ID:8764517459743
and getting a good book on Oracle performance tuning (e.g. "Effective Oracle by Design" by Tom Kyte).
Related
i'm struggeling with Performance in oracle. Situation is: Subsystem B has a dblink to master DB A. on System B a query completes after 15 seconds over dblink, db plan uses appropriate indexes.
If same query should fill a table in a stored procedure now, Oracle uses another plan with full scans. whatever i try (hints), i can't get rid of these full scans. that's horrible.
What can i do?
The Oracle Query Optimizer tries 2000 different possibilities and chooses the best one in normal situations. But if you think it choose wrong plan, You may suspect the following cases:
1- Your histograms which belongs to querying tables are deprecated.
2- Your indexes can not be used because of your faulty query.
3- You can use index hints to force the indexes to be used.
4- You can use SQL Advisor or run TKProf for performance analysis and decide what's wrong or what caused bad performance. Check network, Disk I/O values etc.
If you share your query we can give you more information.
Look like we are not taking same queries in two different conditions.
First case is Simple select over dblink & Second case is "insert as select over dblink".
can you please share two queries & execution plans here as You may have them handy. If its not possible to past queries due to security limitations, please past execution plans.
-Abhi
after many tries, I could create a new DB Plan with Enterprise Manager. now it's running perfect.
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 have an Oracle bind query that is extremely slow (about 2 minutes) when it executes in my C# program but runs very quickly in SQL Developer. It has two parameters that hit the tables index:
select t.Field1, t.Field2
from theTable t
where t.key1=:key1
and t.key2=:key2
Also, if I remove the bind variables and create dynamic sql, it runs just like it does in SQL Developer.
Any suggestion?
BTW, I'm using ODP.
If you are replacing the bind variables with static varibles in sql developer, then you're not really running the same test. Make sure you use the bind varibles, and if it's also slow you're just getting bit by a bad cached execution plan. Updating the stats on that table should resolve it.
However if you are actually using bind variables in sql developers then keep reading. The TLDR version is that parameters that ODP.net run under sometimes cause a slightly more pessimistic approach. Start with updating the stats, but have your dba capture the execution plan under both scenarios and compare to confirm.
I'm reposting my answer from here: https://stackoverflow.com/a/14712992/852208
I considered flagging yours as a duplicate but your title is a little more concise since it identifies the query does run fast in sql developer. I'll welcome advice on handling in another manner.
Adding the following to your config will send odp.net tracing info to a log file:
This will probably only be helpful if you can find a large gap in time. Chances are rows are actually coming in, just at a slower pace.
Try adding "enlist=false" to your connection string. I don't consider this a solution since it effecitively disables distributed transactions but it should help you isolate the issue. You can get a little bit more information from an oracle forumns post:
From an ODP perspective, all we can really point out is that the
behavior occurs when OCI_ATR_EXTERNAL_NAME and OCI_ATR_INTERNAL_NAME
are set on the underlying OCI connection (which is what happens when
distrib tx support is enabled).
I'd guess what you're not seeing is that the execution plan is actually different (meaning the actual performance hit is actually occuring on the server) between the odp.net call and the sql developer call. Have your dba trace the connection and obtain execution plans from both the odp.net call and the call straight from SQL Developer (or with the enlist=false parameter).
If you confirm different execution plans or if you want to take a preemptive shot in the dark, update the statistics on the related tables. In my case this corrected the issue, indicating that execution plan generation doesn't really follow different rules for the different types of connections but that the cost analysis is just slighly more pesimistic when a distributed transaction might be involved. Query hints to force an execution plan are also an option but only as a last resort.
Finally, it could be a network issue. If your odp.net install is using a fresh oracle home (which I would expect unless you did some post-install configuring) then the tnsnames.ora could be different. Host names in tnsnams might not be fully qualified, creating more delays resolving the server. I'd only expect the first attempt (and not subsequent attempts) to be slow in this case so I don't think it's the issue but I thought it should be mentioned.
Are the parameters bound to the correct data type in C#? Are the columns key1 and key2 numbers, but the parameters :key1 and :key2 are strings? If so, the query may return the correct results but will require implicit conversion. That implicit conversion is like using a function to_char(key1), which prevents an index from being used.
Please also check what is the number of rows returned by the query. If the number is big then possibly C# is fetching all rows and the other tool first pocket only. Fetching all rows may require many more disk reads in that case, which is slower. To check this try to run in SQL Developer:
SELECT COUNT(*) FROM (
select t.Field1, t.Field2
from theTable t
where t.key1=:key1
and t.key2=:key2
)
The above query should fetch the maximum number of database blocks.
Nice tool in such cases is tkprof utility which shows SQL execution plan which may be different in cases above (however it should not be).
It is also possible that you have accidentally connected to different databases. In such cases it is nice to compare results of queries.
Since you are raising "Bind is slow" I assume you have checked the SQL without binds and it was fast. In 99% using binds makes things better. Please check if query with constants will run fast. If yes than problem may be implicit conversion of key1 or key2 column (ex. t.key1 is a number and :key1 is a string).
When you write rather complex SQL for Oracle, sooner or later you will have to apply the odd execution hint because Oracle can't seem to figure out the "best" execution plan itself.
http://download.oracle.com/docs/cd/B19306_01/server.102/b14211/hintsref.htm
Now this is certainly not a SQL standard. But still, I'm wondering, are there any other RDBMS that support these kinds of hints, and I really mean hints that are "embedded" in SQL? Are they similar, syntactically (i.e. also placed between the SELECTkeyword and the first selected COLUMN)? Do you know of a general documentation page comparing hints in various RDBMS?
N.B: I'm mostly interested in these RDBMS: Postgres, MySQL, HSQLDB, H2, Derby, SQLite, DB2, Sybase, SQL Server
I know that in db2 the plans are made fixed in some way, not how. In Oracle 11g there are other options besides adding hints to queries. These are SQLProfiles and SQLPlan Baselines, both very powerful. I just finished a performance tuning project where we did not add even a single hint to the code, on the contrary.
You can add Oprimizer Hints to any SQL Server Query
The PLAN clause allows you to define a particular plan to your query in Firebird.
AFAIK, nothing standard nor close to it, but in general, you can do this in a lot of RDBM's, but not all.
I'd also remind you, if you are making some sort of comparison with other DB platforms, that hints in Oracle are entirely non-binding. Which is to say that Oracle is free to disregard your hint if it so chooses.
Hints can be helpfull but I find that I rarely use them anymore - at least not compared to the past when I was working with the older optimizers in earlier Oracle versions. Back then hints were much more of a staple to performance tuning than they are now.
I have created sql tuning task for some query and executed it.After generating report,It recommends me to run the following command:
execute dbms_sqltune.accept_sql_profile(task_name => 'tuning_task_name',
replace => TRUE);
But! Of course I want to know what does this profile do?! and after searching this question on the internet I found the following query:
select
rat.attr1
from
sys.WRI$_ADV_TASKS tsk,
sys.WRI$_ADV_RATIONALE rat
where
tsk.name = 'tuning_task_name'
and rat.task_id = tsk.id
And the result of this query is the following:
I don't understand what does it do :( And also I want to convert this profile hints into the more readable (/+ hint/) sql hints?
Yes, you saw those outline hints.
They will be forced to be used when oracle optimizer parsing this SQL. Hence, the generated execution plan is affected by those hints.
If you want to use similar hints in you SQL, you just need simply copy them into you SQL where follows SELECT, commented by /+/. Be aware the Query Block names, they will be changed in different SQL.
Here is a full list of Oracle SQL hints for your reference.
http://www.hellodba.com/reader.php?ID=208&lang=EN
http://www.hellodba.com/reader.php?ID=220&lang=EN
Hope this helps
Based on page 24 of this PDF it looks like these hints are mostly telling the optimizer to adjust the cardinality estimates for different objects. WHY the profiler is doing this I'm not sure; maybe the statistics are bad or maybe it's making fake statistics to force a better plan? But perhaps a better way to understand the profile is to compare the explain plan before and after.