Oracle _smm_px_max_size Parameter

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Don Burleson Blog

Oracle _smm_px_max_size Parameter

Oracle Database Tips by Donald Burleson

The hidden parameter _smm_px_max_size is used for Oracle parallel query, and defaults to 30% of the pga_aggregate_target setting, divided by degree of parallelism (as set by a PARALLEL hint, the "alter table xxx parallel" command, or the parallel_automatic_tuning initialization parameter).

For example, by default a DEGREE=4 parallel query would have a maximum value of 15 megabytes. Remember, parallel full-table scans bypass the data buffers and store the incoming data rows in the PGA region and not inside the data buffers (as defined by the db_cache_size parameter).

Oracle professional might want to allow individual tasks to exceed the default limits imposed by Oracle. For example, PC-based Oracle servers (1 or 2 CPU's with 8 gigabytes of RAM) will often have unused RAM available. For example, a fully-cached 5 gigabyte database on an 8 gigabyte dedicated Oracle server will have approximately 1 gigabyte available for the PGA (allowing 20% for the OS and other SGA regions).

By re-setting pga_aggregate_target = 1000m, _pga_max_size = 1000m and _smm_px_max_size = 333m, parallel queries may now have up to 330 megabytes of RAM (30% of pga_aggegate_target), such that a DEGREE=4 parallel query would have 83 megabytes (333 meg/4).

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