From: | Hadi Moshayedi <hadi(at)moshayedi(dot)net> |
---|---|
To: | pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Cost estimation in foreign data wrappers |
Date: | 2014-02-21 14:11:53 |
Message-ID: | CAK=1=WoyxQnXzGEbuZr2cRaXgS5fbR1o=cwYV=QRcfg079UKRQ@mail.gmail.com |
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Lists: | pgsql-hackers |
Hello,
There is a callback function in fdw's which should also set estimates for
startup and total costs for each path. Assume a fdw adds only one path
(e.g. in file_fdw). I am trying to understand what can go wrong if we do a
bad job in estimating these costs.
Since we have only one scan path here, it doesn't make a difference in
choosing the best scan path.
By looking at the code and doing some experiments, I think this can
be significant in (1) underestimating a nested loop's cost, (2) not
materializing inner table in nested loop.
* Are there any other cases that this can be significant?
* Assume we are not sure about the exact cost, but we know that it is in
[lower_bound, upper_bound] range, where upper_bound can be 10x lower_bound
Then, what value is better to choose? lower bound? upper bound? or average?
Thanks,
-- Hadi
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