sort_area_retained_size之tom解释

sort_area_retained_size 摘录一段asktom中tom的解释，对sort内存分配的方式进行了描述：

it will allocate up to sort_area_retained_size in the UGA and the spill over into the "pure" PGA up to sort_area_size if the sort exceeds the sort_area_retained_size AND sort_area_size is greater then sort_area_retained_size.

 

the sort_area_retained_size parameter is often misunderstood. At a high level, this is what happens:

 

to do a sort, Oracle allocates chunks of memory up to a maximum of sort_area_size.

 

As each chunk is allocated -- Oracle checks to see if the sort area is less then the sort_area_retained_size.

 

If so, this chunk is allocated in memory that survives from call to call (in the UGA -- where ever the UGA might be -- in the pga or sga). This memory survives as long as needed (across many calls potentially to fetch rows and such)

 

If not, this chunk is allocated in the "pure pga" and freed at the end of the call. This memory does not survive across calls at all.

 

Sort area memory is not necessarily contigous memory -- hence bits of it could be in the SGA, some in the PGA -- it matters not to Oracle.

 

see also

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1) What happens when the sort is completed,I mean where is the resultset(fetched rows) kept before sending it to the user process.According to me ,it is in UGA.

1) the sort workarea is kept in whole or part (the rest goes to TEMP on disk). The rows come from this 'retained' size.

In manual memory management, you used to set

a) sort_area_size
b) sort_area_RETAINED_size

the amount of memory upto (a) would be allocated during the sort and upon completion - right before returning the data, the workarea would be shrunk down to (b) with any excess being written to disk. So, you could set (a) to 10mb and (b) to 1mb - we'd use 10mb to sort the data, shrink the workarea does to 1mb and use that as a buffer to return the data to the client through.

In automatic memory management, the server sets (a) and (b) as it likes.