How to check current and maximum SGA and PGA usage in Oracle

Four short queries against V$SGASTAT, V$SGAINFO, and V$PGASTAT for the four numbers you actually want when someone asks how much memory Oracle is using right now.

-- Current SGA size
SELECT SUM(bytes / 1024 / 1024) AS current_sga_size_mb
FROM   v$sgastat;

-- Current PGA in use
SELECT (value / 1024 / 1024) AS current_pga_in_use_mb
FROM   v$pgastat
WHERE  name = 'total PGA inuse';

-- Maximum SGA size
SELECT SUM(bytes / 1024 / 1024) AS max_sga_size_mb
FROM   v$sgainfo
WHERE  name = 'Maximum SGA Size';

-- Maximum PGA allocated
SELECT (value / 1024 / 1024) AS max_pga_allocated_mb
FROM   v$pgastat
WHERE  name = 'maximum PGA allocated';