Tom Lane wrote:
Gregory Stark <[EMAIL PROTECTED]> writes:
"Manoel Henrique" <[EMAIL PROTECTED]> writes:
Yes, I'm relying on the assumption that backwards scan has the same cost as
forward scan, why shouldn't it?
G...we expect that forward scans will result
in the kernel doing read-ahead, ...
A backwards scan will get no such overlapping and thus be up to 2X
slower, unless the kernel is smart enough to do read-ahead for
descending-order read requests. Which seems not too probable.
Linux's old adaptive readahead patches claimed to[1]:
It also have methods to detect some less common cases:
- reading backward"
Interestingly the author of that patch used postgres as the example
application that benefits from the patch (30%).
I'm not sure if the backward reading feature got kept
in the simplified on-demand readahead that seems to have
superseded the adaptive readahead stuff in 2.6.23[2].
[1] http://lwn.net/Articles/185469/
[2]
http://kernelnewbies.org/Linux_2_6_23#head-102af265937262a7a21766ae58fddc1a29a5d8d7
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