On Sat, Jan 9, 2016 at 11:02 AM, Tomas Vondra <tomas.von...@2ndquadrant.com> wrote: > Which means the "dim.r" column has 100 different values (0-99) with uniform > distribution. So e.g. "WHERE r < 15" matches 15%.
I think that the use of a uniform distribution to demonstrate this patch is a bad idea, unless you want to have a conversation about the worst case. Look at the use cases for bloom filters in general. They're almost all some variation on the same theme: checking a bloom filter inexpensively, to avoid an expensive cache miss (of some fashion). The Google Chrome web browser uses a bloom filter for malicious URLs. It usually avoids consulting Google's servers about whether or not any given URL that is visited is malicious by using the bloom filter. This is effective in large part because the top 100 websites ranked by popularity have a huge falloff in popularity as you go down the list. It looks like a Zipfian distribution, and so I imagine they get pretty far with a very small bloom filter, even though in theory the chances of any given valid URL resulting in a cache miss is very high. Generally, uniform distributions are rare in a non-canonical list of things, like a hash join outer relation's attribute. -- Peter Geoghegan -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
