> Now I've asked for the quickest path to detailed > understanding of the pg IO subsystem. The goal being to get > more up to speed on its coding details. Certainly not to > annoy you or anyone else.
Basically pg does random 8k (compile time blocksize) reads/writes only. Bitmap and sequential scans read 8k blocks in order. Only WAL does n x 8k writes with one system call. pg relys on the OS readahead (== larger block IO) to do efficient IO. Basically the pg scan performance should match a dd if=file of=/dev/null bs=8k, unless CPU bound. Andreas ---------------------------(end of broadcast)--------------------------- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match