Re: [HACKERS] [POC] A better way to expand hash indexes.

On Fri, Feb 17, 2017 at 7:21 PM, Mithun Cy <mithun.cy@enterprisedb.com> wrote:
>
> To implement such an incremental addition of bucket blocks I have to
> increase the size of array hashm_spares in meta page by four times.
> Also, mapping methods which map a total number of buckets to a
> split-point position of hashm_spares array, need to be changed. These
> changes create backward compatibility issues.
>

How will high and lowmask calculations work in this new strategy?
Till now they always work on doubling strategy and I don't see you
have changed anything related to that code.  Check below places.

_hash_metapinit()
{
..
/*
* We initialize the index with N buckets, 0 .. N-1, occupying physical
* blocks 1 to N.  The first freespace bitmap page is in block N+1. Since
* N is a power of 2, we can set the masks this way:
*/
metap->hashm_maxbucket = metap->hashm_lowmask = num_buckets - 1;
metap->hashm_highmask = (num_buckets << 1) - 1;
..
}

_hash_expandtable()
{
..
if (new_bucket > metap->hashm_highmask)
{
/* Starting a new doubling */
metap->hashm_lowmask = metap->hashm_highmask;
metap->hashm_highmask = new_bucket | metap->hashm_lowmask;
}
..
}

Till now, we have worked hard for not changing the page format in a
backward incompatible way, so it will be better if we could find some
way of doing this without changing the meta page format in a backward
incompatible way.  Have you considered to store some information in
shared memory based on which we can decide how much percentage of
buckets are allocated in current table half?  I think we might be able
to construct this information after crash recovery as well.

-- 
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com