Re: store narrow values in hash indexes?

On Sat, Sep 24, 2016 at 10:33:01AM +0530, Amit Kapila wrote:
> On Sat, Sep 24, 2016 at 1:02 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> > Currently, hash indexes always store the hash code in the index, but
> > not the actual Datum. It's recently been noted that this can make a
> > hash index smaller than the corresponding btree index would be if the
> > column is wide. However, if the index is being built on a fixed-width
> > column with a typlen <= sizeof(Datum), we could store the original
> > value in the hash index rather than the hash code without using any
> > more space. That would complicate the code, but I bet it would be
> > faster: we wouldn't need to set xs_recheck, we could rule out hash
> > collisions without visiting the heap, and we could support index-only
> > scans in such cases.
> >
>
> What exactly you mean by Datum? Is it for datatypes that fits into 64
> bits like integer. I think if we are able to support index only scans
> for hash indexes for some data types, that will be a huge plus.
> Surely, there is some benefit without index only scans as well, which
> is we can avoid recheck, but not sure if that alone can give us any
> big performance boost. As, you say, it might lead to some
> complication in code, but I think it is worth trying.
>
> Won't it add some requirements for pg_upgrade as well?

Yes, pg_upgrade will mark the indexes as invalid and supply a script to
reindex them.

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Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

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