On Mon, Mar 23, 2015 at 6:01 AM, Rajeev rastogi
<rajeev.rastogi@huawei.com> wrote:
> The cost of re-scan of SeqScan node is considered to be same scan of SeqScan
> node, which always assumes that the records is fetched from disk and hence
> disk access cost is added (As we don’t know really how much memory will be
> available to cache during execution).
That's a general problem not limited to materialize nodes. We might
choose to do a heap-sort rather than a quick-sort, but it may turn out
that the "tapes" we create end up in the OS buffer cache instead of on
physical storage; in fact, it's probably the common case. Scans are
costed using seq_page_cost and random_page_cost, but most of the time
the "random" page cost will not be the cost of a head seek, because
we'll find the data in the OS page cache. Some of the time it really
will be a head seek, but we have no idea whether that will happen in
any given case. The autovacuum cost delays have this problem too: a
"miss" in shared buffers may really be a hit in the OS page cache, but
we don't know.
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