On Aug 31, 2016, at 3:01 PM, Bobby Mozumder <bmozumder@xxxxxxxxx> wrote:
Postgres knows the number of rows it will need to pull to do your query, but it has no way of knowing if a block not in its own cache can be satisfied via filesystem cache, or if it will fall through to disk read. If you are on linux, you might be able to tell the effectiveness of your filesystem cache via something like http://www.brendangregg.com/blog/2014-12-31/linux-page-cache-hit-ratio.html …but that's hardly going to show you something as granular as a per-query cost. |
