Re: Ambigous Plan - Larger Table on Hash Side

On 2018-03-12 12:52:00 -0400, Tom Lane wrote:
> Narendra Pradeep U U <narendra(dot)pradeep(at)zohocorp(dot)com> writes:
> > Recently I came across a case where the planner choose larger table on hash side. I am not sure whether it is an intended behavior or we are missing something.
>
> Probably the reason is that the smaller table has a less uniform
> distribution of the hash key. You don't want to hash with a nonuniform
> distribution of the hashtable key; if many keys go into the same bucket
> then performance degrades drastically.

Not sure I follow. Unless the values are equivalent (i.e. duplicate key
values), why should non-uniformity in key space translate to hash space?
And if there's duplicates it shouldn't hurt much either, unless doing
a semi/anti-join? All rows are going to be returned and IIRC we quite
cheaply continue a bucket scan?

Greetings,

Andres Freund