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The limitations of the pga_aggregate_target parameter

Pachot Franck has published an excellent overview of the limitations of the pga_aggregate_target parameter:

http://www.dba-village.com/village/dvp_tips.TipDetails?TipIdA=2844

Even if you set PGA_AGGREGATE_TARGET to high values (gigabytes) the memory allowed for one process is limited by undocumented parameter _pga_max_size that is set to 200Mb for every instance.

Thus, queries cannot use more than 200Mb workarea for all of their sorts or hash.

Moreover, each sort cannot use more than _smm_max_size that is calculated from the lowest from:

- _pga_max_size / 2

- or 5% pga_aggregate_target

Then by default and whatever pga_aggregate_target is, one sort cannot use more than 100Mb memory for a serial process.

For parallel operations, limit is _smm_px_max_size which defaults to 30% pga_aggregate_target divided by degree of parallelism.

Mr. Franck also notes that you can add more RAM for sorting and hash joins by adjusting the hidden parameter _pga_max_size up to 1Gb ... with oracle support agreement!
 

 

 
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