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Different between SGA_TARGET and TOTAL SGA


What is the difference between SGA_TARGET (in V$PARAMETER) and Total SGA (the sum of all values in V$SGA) ?

For the SGA_TARGET, I run : SELECT name,value,display_value,isdefault,isbasic FROM V$PARAMETER WHERE name IN ('memory_target', 'sga_target', 'pga_aggregate_target'); I get around 14 GB.

For the total SGA, I run : SELECT sum(value)/1024/1024 "TOTAL SGA (MB)" FROM v$sga; I get around 26 GB. The query on SGA size (SELECT name,value/1024/1024 “SGA (MB)” “FROM v$sga;) gives out result as follows :

From, it says "SGA_TARGET specifies the total size of all SGA components".

If SGA_TARGET is the total of all SGA, then why do they differ ?

Thanks for clearing up my confusion.

I am using Oracle 11.2g.