Overview

There is much to be said for not using various ratios as tuning indicators, in fact for not using any of them. Many of these ratios can conceal some very large and ugly numbers since they divide one number by another. The denominator, or the number used to divide by, simply disappears. Ratios can be useful to indicate a possible problem or problems. Well in fact a low or high ratio could indicate a possible problem but it could also conceal a possible problem. Additionally a ratio could indicate no problem at all. Ratios are best used as possible symptoms of possible problems. "Hmmmm "

Ratios have received a lot of bad press recently. Much of this is founded in truth. Ratios should not be completely ignored. I find even the database buffer cache hit ratio useful. However, I do not necessarily think it is a bad thing if it is low or high. So what! That could be because of application behavior. A possible mistake made with ratios in the past was that an unacceptable value was assumed to be tunable. A ratio is not tunable. However, it can indicate that something else could be tunable such as an ugly SQL statement in an application full of very well-tuned SQL code. If the database buffer cache is "doing something weird" like going up and down say between 60% and 90% every 5 s then something odd may be going on. Increasing the size of the database buffer cache will...