[Bizgres-general] Default work_mem

[Luke Lonergan]

I'm planning to set the default work_mem setting for the binary installers
of Bizgres to 64MB to ensure that reasonable sort performance is obtained.
The knee of the sorting performance curve is currently pretty steep, as
shown in this graph of sort time for one column of a 12GB table:



As you can see from the blue line in the graph, the performance of sorting
with less than 64MB of work_mem sharply increases.  This is case-specific,
following a relationship of work_mem = sqrt(sort_set_size) as a general
trend.  Given that most business intelligence case sort large amounts of
data, 64MB seems like a good starting point with the current sort code.

The risk is that overallocation of work_mem in high concurrency environments
can cause machines to run out of memory.

- Luke 
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