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Indexes -- Basics

Indexes provide faster access to rows. Indexes can be created on any column. Usually search on a column is faster if an index exists on that column.

Indexes are faster means to access database objects. By this defination it is clear that indexes are or
should be used for performance enhancements. But this is not always true, using indexes will not always help
in improving the performance.


Indexes are always independent of the physical or logical data/objects or even the database, meaning creating

and deleting(dropping) an index will harm the data or the functionality of your code. It only affects your
performance.

Though indexes are independent they require storage space.


I always get queries from my customers that we have an index but my database doesnt use it. I have created an index but after creating it my queries are slow.


SYNTAX:
CREATE INDEX index_name ON table_name (column_name);
EXAMPLE:
CREATE INDEX anyname ON emp (sal);
DROPPING INDEX:
DROP INDEX anyname;

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SQL> startup mount pfile='/tmp/initdlfasp12.ora'
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Linux-x86_64 Error: 28: No space left on device


This as you can see is on Linux x86 with 64 bit processor. We got this error after we changed SGA on 10gR2 database. So was sure that this is something to do with the OS.

Parameters to check for this are shmall.

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$cat /proc/sys/kernel/shmmax
53687091200

$ getconf PAGE_SIZE
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As per Oracle SHMALL should be set to the total amount of physical RAM divided by page size.

Our system has 64GB memory, so change kernel.shmall = 1024 * 1024 * 1024 * 64 / 4096 = 16777216


Once this value is calculated you can modify Linux system configuration file directly.

$ su - root
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