Question: What is the Row Cache Hit Ratio in
V$SYSMETRIC_HISTORY? Is it the dictionary cache hit ratio?
Answer: The row cache hit ratio is
an a overall row caching metric for the library cache
(shared_pool_size)!
The row cache hit ratio relates
to the total number of bytes processed in the related the
total number of bytes processed plus extra bytes
read/written in extra passes.
The Row Cache
Hit Ratio is derived from this query for
V$SYSMETRIC_HISTORY:
select
(sum(getmisses)/SUM(gets))*100 row_cache_hit_ratio
from
v$rowcache;
Oracle notes that v$rowcache displays statistics for data
dictionary activity. Each row contains statistics for one
data dictionary cache!
With regarding to the sizing or the row cache miss ratio,
Oracle recommends that it:
If free memory is close to zero
AND EITHER
the library cache hit ratio
is less than 0.95
OR
the row
cache hit ratio is less than 0.95
THEN increase the
shared_pool_size parameter until the ratios stop improving.
The row cache hat ratio is probably used in automatic
memory management (AMM) when re-sizing the shared pool.
