Re: Hash Indexes

On 2016-09-21 22:23:27 -0400, Tom Lane wrote:
> Andres Freund <andres@anarazel.de> writes:
> > Sure. But that can be addressed, with a lot less effort than fixing and
> > maintaining the hash indexes, by adding the ability to do that
> > transparently using btree indexes + a recheck internally.  How that
> > compares efficiency-wise is unclear as of now. But I do think it's
> > something we should measure before committing the new code.
> 
> TBH, I think we should reject that argument out of hand.  If someone
> wants to spend time developing a hash-wrapper-around-btree AM, they're
> welcome to do so.  But to kick the hash AM as such to the curb is to say
> "sorry, there will never be O(1) index lookups in Postgres".

Note that I'm explicitly *not* saying that. I just would like to see
actual comparisons being made before investing significant amounts of
code and related effort being invested in fixing the current hash table
implementation. And I haven't seen a lot of that.  If the result of that
comparison is that hash-indexes actually perform very well: Great!


> always be superior, I don't see how it follows that we should refuse to
> commit work that's already been done.  Is committing it somehow going to
> prevent work on the btree-wrapper approach?

The necessary work seems a good bit from finished.


Greetings,

Andres Freund