Re: gaussian distribution pgbench
От | Fabien COELHO |
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Тема | Re: gaussian distribution pgbench |
Дата | |
Msg-id | alpine.DEB.2.10.1403150738110.13791@sto обсуждение исходный текст |
Ответ на | Re: gaussian distribution pgbench (Heikki Linnakangas <hlinnakangas@vmware.com>) |
Ответы |
Re: gaussian distribution pgbench
Re: gaussian distribution pgbench Re: gaussian distribution pgbench |
Список | pgsql-hackers |
Hello Heikki, > A couple of comments: > > * There should be an explicit "\setrandom ... uniform" option too, even > though you get that implicitly if you don't specify the distribution Indeed. I agree. I suggested it, but it got lost. > * What exactly does the "threshold" mean? The docs informally explain that > "the larger the thresold, the more frequent values close to the middle of the > interval are drawn", but that's pretty vague. There are explanations and computations as comments in the code. If it is about the documentation, I'm not sure that a very precise mathematical definition will help a lot of people, and might rather hinder understanding, so the doc focuses on an intuitive explanation instead. > * Does min and max really make sense for gaussian and exponential > distributions? For gaussian, I would expect mean and standard deviation as > the parameters, not min/max/threshold. Yes... and no:-) The aim is to draw an integer primary key from a table, so it must be in a specified range. This is approximated by drawing a double value with the expected distribution (gaussian or exponential) and project it carefully onto integers. If it is out of range, there is a loop and another value is drawn. The minimal threshold constraint (2.0) ensures that the probability of looping is low. > * How about setting the variable as a float instead of integer? Would seem > more natural to me. At least as an option. Which variable? The values set by setrandom are mostly used for primary keys. We really want integers in a range. -- Fabien.
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