Tuning the buffer cache hit ratio
A logical read occurs whenever a user requests data from the database.
It occurs whenever the data is in the buffer cache or whether the user process
must read it from disk. If the data must be read from disk then a physical
read occurs. Oracle keeps track of logical and physical reads in the
Use the following SQL statement to determine the values required for the hit radio calculation:
The cache-hit ratio can be calculated as follows:
If the cache-hit ratio goes below 90% then:
I am grateful to Kamil Jakubovic for provided me with a single statement for calculating the cache hit ratio:
These notes have been tested against Oracle 7.3.4, Oracle 8.0.5, Oracle 8.1.5 and Oracle 9.0.1 and may apply to other versions as well.
This page represents one page of many pages of notes I made whilst working with Oracle 7 through to 10i. I now no longer work with Oracle databases, but hope that these notes will continue to be of benefit to others.
About the author: Brian Cryer is a dedicated software developer and webmaster. For his day job he develops websites and desktop applications as well as providing IT services. He moonlights as a technical author and consultant.