SGA Summary

When the Oracle program runs, it grabs a large chunk of memory in which it can do its work. That 'chunk', in its entirety, is known as the System Global Area, or SGA.

The SGA is not a monolithic chunk of memory, however: it has internal structure, and is internally chopped up into discrete areas of memory which perform specialised tasks. These sub-areas are known as the Shared Pool (which caches execution plans and the data dictionary tables), the Buffer Cache (which caches ordinary data), the Log Buffer (which records changes made to ordinary data), the Large Pool (which is needed for efficient memory allocations to parallel slaves and backup processes), the Java Pool (providing memory to Java stored procedures) and the Streams Pool (unique to 10g and above, and providing memory to replication services).