Dne 24.12.2010 13:15, t...@fuzzy.cz napsal(a): >> 2010/12/24 Florian Pflug <f...@phlo.org>: >> >>> On Dec23, 2010, at 20:39 , Tomas Vondra wrote: >>> >>>> I guess we could use the highest possible value (equal to the number >>>> of tuples) - according to wiki you need about 10 bits per element >>>> with 1% error, i.e. about 10MB of memory for each million of >>>> elements. >>> >>> Drat. I had expected these number to come out quite a bit lower than >>> that, at least for a higher error target. But even with 10% false >>> positive rate, it's still 4.5MB per 1e6 elements. Still too much to >>> assume the filter will always fit into memory, I fear :-( >> >> I have the impression that both of you are forgetting that there are 8 >> bits in a byte. 10 bits per element = 1.25MB per milion elements. > > We are aware of that, but we really needed to do some very rough estimates > and it's much easier to do the calculations with 10. Actually according to > wikipedia it's not 10bits per element but 9.6, etc. But it really does not > matter if there is 10MB or 20MB of data, it's still a lot of data ...
Oooops, now I see what's the problem. I thought you were pointing out something out, but I've actually used 1B = 1b (which is obviously wrong). But Florian already noticed that and fixed the estimates. Tomas -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers