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Don Burleson Blog 









Cache Oracle DUAL - table caching

Don Burleson


Many Oracle databases have high consistent gets (and hence, high CPU consumption) from excessive "select 1 from dual; calls. 

This issue is fixed in Oracle10g with the new "fast dual" execution plan, but you can do a trick in earlier releases of Oracle to reduce consistent gets (logical I/O) from three to zero.

There are two ways to cache the dual fixed table in Oracle; this, take your pic:

Fix approach One:

connect sys/mypass as sysdba;
create view x_$dual as select * from x$dual ;

grant select on x_$dual to public;

connect my_schema_owner/mypassword; 

create synonym DUAL for sys.x_$dual ;

Fix approach Two:

1. Allocate a small keep pool if it doesn’t exist.

2. From the sys user issue “alter table dual cache storage(buffer_pool keep);

3. This will drop the consistent gets from accessing dual from 3 to 0.

4. The problem is dual is always an FTS so it is placed at the end of the LRU and is aged out fast. By marking it as cached and placing it in the Keep pool this is avoided. In order to get to 0 CR  I had to use both cache and the keep option to make this work.

This caching of the dual fixed table can greatly reduce CPU consumptions of certain applications.




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