On Wed, Apr 30, 2014 at 11:19 AM, Peter Geoghegan <p...@heroku.com> wrote:
> On Wed, Apr 30, 2014 at 11:03 AM, Jeff Janes <jeff.ja...@gmail.com> wrote: > > If we don't put in the work to make them useful, then they won't ever > become > > useful. > > > > If we do put in the effort (and it would be considerable) then I think > they > > will be. But you may be correct that the effort required would perhaps > be > > better used in making btree even more better. I don't think we can > conclude > > that definitively without putting in the work to do the experiment. > > My argument doesn't hinge on there being more important work to do. > Rather, I simply don't think that there is never going to be a > compelling reason to use hash indexes in production. I have an indexed text column with an average length of 50, a stddev length of 15, and a pronounced right skew. Currently, the longest value in it is 811. But inevitably someone will need to insert something longer than 2712. When that day comes, I will drop the btree index and add a hash index (unless we remove that limitation from btree indexes in the mean time). It lets me sleep at night knowing that I have that option today, even if it would complicate crash recovery. Apart from the > obvious inflexibility, consider what it takes to make index creation > fast - insertion-style building of indexes is much slower. Consider > multi-key indexes. > I'm pretty sure hash indexes already implement a bulk creation fast path. In any case, I've never noticed them being slow, and I've tested some pretty big ones. > Now, I'm not telling anyone what to work on, and if someone wants to > make hash indexes WAL-logged to plug that hole, don't let me stop you. > It probably makes sense as a project to learn more about Postgres > internals. However, it would be unfair to not speak up given my > misgivings around the practical utility of hash indexes. > Sure, and we all have our own opinions on that. Should we summarize them somewhere easier to follow than a long email thread but more detailed than a TODO entry? Whatever happened with the GSOC people? That should be well under way by now, is anyone working on it? Are the discussions of their efforts on-list, or is it between them and their mentors? Cheers, Jeff
