shared_buffers is the amount of space postgresql can use as temp memory space to put together result sets. It is not intended as a cache, and once the last backend holding open a buffer space shuts down, the information in that buffer is lost. If you're working on several large data sets in a row, the buffer currently operates FIFO when dumping old references to make room for the incoming data.
Contrast this to the linux or BSD kernels, which cache everything they can in the "spare" memory of the computer. This cache is maintained until some other process requests enough memory to make the kernel give up some of the otherwise unused memory, or something new pushes out something old.
Do checkpoints operate on the Postgres-managed buffer, or the kernel-managed cache?
Jack Orenstein
---------------------------(end of broadcast)--------------------------- TIP 9: the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
