Buffer Cache
The Oracle SGA region that caches database blocks in memory to reduce repeated physical reads from datafiles.
Oracle Runtime Context
The Oracle buffer cache stores database blocks in the SGA so repeated access can be satisfied as logical reads without another physical read from the datafile. Distinguishing logical I/O from physical I/O is fundamental when diagnosing SQL and memory behavior.
Measurement Boundary
A high cache-hit ratio is not proof of good performance, and a larger cache does not repair a query that visits far too many blocks. CPU, latch or mutex work, access paths, latency, and the actual I/O profile must be considered together.