Monday, February 23, 2009

Find Top N resource absorbing SQL Queries

At times database performance problems arise that require your diagnosis and correction. Sometimes problems are brought to your attention by users who complain about slow performance. Other times you might notice performance spikes in the Host CPU chart on the home page. Suppose user reported slow performance and this occurred in night or in holiday. Now You need to see which sql queries were run at that specific time along with their resources usages details.

With default settings, in every hour a snapshots taken by Automatic Workload Repository (AWR) and those snapshots are available for next 7 days. Here is a SQL query that give the desired top N resource sensitive queries :

BEGIN_INTERVAL_TIME >= TO_DATE('18-feb-2009 18:00', 'dd-mon-yyyy hh24:mi')
AND END_INTERVAL_TIME <= TO_DATE('18-feb-2009 18:40', 'dd-mon-yyyy hh24:mi'))
) WHERE rownum < =200;

Wednesday, February 11, 2009


We all are know about Oracle Data Base Link and I called it dblink in short throughout this article. Usisg dblink you can easily access remote Database Object. Now Please think the following situation:


Suppose you have 2 tables: 1. table1 in local DB 2. table2 in remote DB.The table1 size is 10 MB and table2 size is 100 MB. We need to join those two tables and we access only in local DB.

If you now think about performance, it is important to where the sql query executed and the join perform. For above situation, it is better to bring table1 from local DB to Remote DB and execute the sql in remote server and finally bring back the result in local DB.

Lets do the job

The DRIVING_SITE hint forces query execution to be done at a different site than that selected by Oracle. This hint can be used with either rule-based or cost-based optimization. The syntax of this hint is:


where table is the name or alias for the table at which site the execution should take place.


SELECT /*+DRIVING_SITE(table2)*/ * FROM tabl1, table2@remote_db
WHERE table1.DEPTNO = table2.DEPTNO;

If this query is executed without the hint, rows from table2 will be sent to the local site and the join will be executed there. With the hint, the rows from table1 will be sent to the remote site and the query will be executed there, returning the result to the local DB.

create Foreign key On two diffrent schema table

Some times you may face easy questions which can put into difficulties. Today I got that kind of a question and i am happy so that I get the answer though it take some time find it.

Here is the question which i got from a forum member :

" how to link a table from scott user to the hr user using a foreign key. For an example hr.emp to scott.dept using a foreign key "

In my answer, I assume hr.emp table has a colunm name scott_dept_id . So I create a foreign key on hr.emp (scott_dept_id) by referencing scott.dept (id).

sql> connect scott / pass;
sql> GRANT select,update, delete, references on dept to hr;

Though only references privilege required to create the foreign key

sql> connect hr/password;
sql> alter table emp add constraint fk_test foreign key (scott_dept_id) references scott.dept (id);

So simple ! but hard enough to bother you, if you not know exactly . Thanks to that forum user who lead me to learn this thing .

Tuesday, February 10, 2009

Reverse Key Index

I think we are familiar with Oracle Index, specially Bitmap Index. Indexes are used to help oracle retrieve data faster. But there is a drawback of Index, if an index create on a large table which under go massive insert/ update, the index may raise contention issue. To get ride from this Oracle provides couple of solutions and Reverse key Index is one of them.

Real world case scenario

Case 1: Suppose table 'track_user' contains login related information of a system. The primary key column 'ID' of the table populated by an increasing sequence. So every new entries (for the index) come to the same blocks when a row inserted into the table. This is the way contention may increase!

As all we know that the primary key constraint is impose on column by creating an unique index on that column. So if we use reverse key indexes in this case then the index entries will go to different blocks and contention will be reduced.

Case 2: If you have a table with column which is populated by an increasing sequence and some times it go under some delete operation for old records. A index was created on That column and this index is on face range scan when you issue a select on that table. But this index has contention issues on index blocks.

To avoide the contention issue you can use Reverse key index as a soluation.

What is Reverse key index ?

Reverse key index was first introduce in Oracle 8. A reverse key index reverses the bytes of each column indexed (except for the ROWID) while keeping the column order same as normal index.


  1. Reverse key index can help avoid performance degradation in indexes in an Oracle Parallel server environment where modifications to the index are concentrated on a small set of leaf blocks. For example, if you insert rows with keys 101, 102 and 103 into a table with a regular index, the rows are likely to be inserted into the same leaf block. When users connected to different instances of the same database insert into the same block a pingoccurs.(When a block is written to disk by one instance so that another instance can read it, it is called a ping.) Excessive pinging will severely degrade performance, so you want to reduce it. In a Reverse Key Index the keys in our example become 101, 201 and 301, and the rows are inserted into disparate parts of the index segment. This type of index is therefore useful in a parallel server environment because it reduces pinging.
  2. Reverse Key Indexes can also be useful in situations where users insert ascending values and delete lower values from a table. A regular index would become skewed, but a Reverse Key Index would not.


With a Reverse Key Index you cannot run an index range scanning query. This is because lexically adjacent keys are not stored next to each other in a Reverse Key Index. You can only perform fetch-by-key value or full-index scans . Of course, you can avoid the index and perform full table scans or use the parallel query option.

Create And Manage Reverse Key Index

You create a Reverse Key Index with the key word REVERSE:

Create Index index_name on table_name (a,b,c) Reverse;

You can rebuild a Reverse Key Index into a regular index with the keyword NOREVERSE

Alter Index index_name Rebuild Noreverse;

If you rebuild a Reverse Key Index without the keyword NOREVERSE, it will rebuilt the Reverse Key Index.

Alter Index index_name Rebuild;

You cannot rebuild a normal index into a reverse key index. You must drop the normal index and create the reverse index.

Sunday, February 1, 2009

Find Bind variable value

The usages of bind variable increase SQL query performance. Suppose you have a SQL query that execute several times and only change are in literal values, IN such case professionals are suggested to use bind variable rather than hard cote value. The impact is significant on query performance if you use bind variable.

But the problem is rice when you try to collect full query text from database, the value of the bind variable is not present in the query text. But you may need those values when you are working on that SQL query specially when you try to tune that query.

Here are two query that help you to find the valu of bind variable. The only thing you needed is the SQL ID

1. If your query was run within 30 minutes:

FROM v$sql_bind_capture WHERE sql_id='';

2. If your query was run more than 30 minutes ago. For this you also need a snapshot Id. By default oracle store snapshots of last 7 days:


Note: v$sql_bind_capture and DBA_HIST_SQLBIND stors only those bind values
which are given in HWERE clause and they do not store timstamp,lob,clob type value

Related Topics:

  1. Bind Variable