_PGA_MAX_SIZE
The parameter _PGA_MAX_SIZE limits the maximum size of all work areas for a single process.
该参数限制的是单个进程所能使用的最大内存。
_SMM_MAX_SIZE
The parameter _SMM_MAX_SIZE limits the maximum size of an individual work area for a single
process.
该参数限制的是单个进程的单个排序操作所能使用的最大内存，一个进程可能有多个排序操作。
如果一个进程有n个排序操作，则n*_SMM_MAX_SIZE<=_PGA_MAX_SIZE
When using automatic PGA memory management, three hidden parameters—_PGA_MAX_SIZE,
_SMM_MAX_SIZE, and _SMM_PX_MAX_SIZE—work behind the scenes to enforce restrictions on
memory consumption. The parameter _PGA_MAX_SIZE limits the size of all work areas in use by
a single process. The size of an individual work area is limited by _SMM_MAX_SIZE for both serial
and parallel execution. When parallel execution is used, an additional restriction on the total
size of all work areas in use by the processes involved is in place. This limit is controlled with
the parameter _SMM_PX_MAX_SIZE. Within certain limits, all three parameters are recalculated at
runtime as a result of modifying PAT. All three parameters may be set manually to override the
result of this calculation.
_SMM_PX_MAX_SIZE限制的是并行执行时所使用的最大内存大小，当执行并行操作时，单个排序操作所能使用的最大内存受_SMM_MAX_SIZE限制，同时所有并行进程的所有排序操作所能使用的内存总和受_SMM_PX_MAX_SIZE参数的限制
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