--On Friday, July 21, 2023 3:32 PM +0000 Maud Parratt
<Maud.Parratt@bjss.com> wrote:

>
>
>
> why? to be clear I'm experimenting on a smaller ephemeral test instance
> with non prod data, so there's no risk to trying things.

The man page spells it out pretty clearly:

     index_hash64 { on | off }
              Use a 64 bit hash for indexing. The default is  to  use  32
bit
              hashes.   These  hashes  are  used  for  equality  and
substring
              indexing. The 64 bit  version  may  be  needed  to  avoid
index
              collisions  when  the  number  of  indexed  values  exceeds
~64
              million. (Note that substring indexing generates multiple
index
              values  per  actual attribute value.)  Indices generated with
32
              bit hashes are incompatible with the 64 bit  version,  and
vice
              versa.  Any  existing  databases  must  be  fully  reloaded
when
              changing this setting. This directive is only  supported  on
64
              bit CPUs.

I'm assuming that you have substring indices, and given the large size of
your database you may start having hash collissions.

You most likely also want to configure olcBkMdbIdlExp, sortvals, and
multival if you haven't.

--Quanah