explanation of some configs

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Hi

I am having some problem understanding the details of a couple of configuration directives. Here is what I think I understand it so far:

effective_cache_size
	- specifies in how many B/KB/MB will be used to cache data
	  between pg and the OS?
	- what is actually cached and how it is actually used by the
          planner and how does it affect the planner and the queries?
	  -in other words, when and why do I need to set this parameter?

checkpoint_segments
	- specifies the number of segments?
	  size: number*16MB?
	- means when number of WAL segments has filled up and matches
          checkpoint segment, dirty data is written to disk?
	- so it specifies how much data is stored in memory/wal before
          written to disk and therefore affects recoverability?

regards

thomas

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