How to check the data dictionary cache hit ratio in Oracle

Aggregate gets and misses across every row-cache slot to get a single hit-ratio number. Below ~95% suggests SHARED_POOL may be undersized, but as always with hit ratios, look at wait events (row cache lock, library cache lock) before touching parameters.

SELECT SUM(gets)                                        AS gets,
       SUM(getmisses)                                   AS misses,
       (1 - (SUM(getmisses) / SUM(gets))) * 100         AS "CACHE HIT RATIO"
FROM   gv$rowcache;