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Don Burleson Blog 







row cache hit ratio Tips

Oracle Database Tips by Donald BurlesonMarch 22, 2015

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:

  (sum(getmisses)/SUM(gets))*100 row_cache_hit_ratio

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
   the library cache hit ratio is less than 0.95
   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.


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