Re: [HACKERS] Write Ahead Logging for Hash Indexes

Robert Haas <robertmhaas@gmail.com> writes:
> On Tue, Mar 14, 2017 at 2:14 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> Robert Haas <robertmhaas@gmail.com> writes:
>>> It's become pretty clear to me that there are a bunch of other things
>>> about hash indexes which are not exactly great, the worst of which is
>>> the way they grow by DOUBLING IN SIZE.

>> Uh, what?  Growth should happen one bucket-split at a time.

> Technically, the buckets are created one at a time, but because of the
> way hashm_spares works, the primary bucket pages for all bucket from
> 2^N to 2^{N+1}-1 must be physically consecutive.  See
> _hash_alloc_buckets.

Right, but we only fill those pages one at a time.

It's true that as soon as we need another overflow page, that's going to
get dropped beyond the 2^{N+1}-1 point, and the *apparent* size of the
index will grow quite a lot.  But any modern filesystem should handle
that without much difficulty by treating the index as a sparse file.

There may be some work to be done in places like pg_basebackup to
recognize and deal with sparse files, but it doesn't seem like a
reason to panic.
        regards, tom lane