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

On Sat, Mar 25, 2017 at 10:13 AM, Mithun Cy <mithun(dot)cy(at)enterprisedb(dot)com> wrote:
> On Tue, Mar 21, 2017 at 8:16 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>> Sure, I was telling you based on that. If you are implicitly treating
>> it as 2-dimensional array, it might be easier to compute the array
>>offsets.
>
> I think calculating spares offset will not become anyway much simpler
> we still need to calculate split group and split phase separately. I
> have added a patch to show how a 2-dimensional spares code looks like
> and where all we need changes.
>

I think one-dimensional patch has fewer places to touch, so that looks
better to me. However, I think there is still hard coding and
assumptions in code which we should try to improve.

1.
+ /*
+ * The first 4 bucket belongs to first splitpoint group 0. And since group
+ * 0 have 4 = 2^2 buckets, we double them in group 1. So total buckets
+ * after group 1 is 8 = 2^3. Then again at group 2 we add another 2^3
+ * buckets to double the total number of buckets, which will become 2^4. I
+ * think by this time we can see a pattern which say if num_bucket > 4
+ * splitpoint group = log2(num_bucket) - 2
+ */

+ if (num_bucket <= 4)
+ splitpoint_group = 0; /* converted to base 0. */
+ else
+ splitpoint_group = _hash_log2(num_bucket) - 2;

This patch defines split point group zero has four buckets and based
on that above calculation is done. I feel you can define it like
#define Buckets_First_Split_Group 4 and then use it in above code.
Also, in else part number 2 looks awkward, can we define it as
log2_buckets_first_group = _hash_log2(Buckets_First_Split_Group) or
some thing like that. I think that way code will look neat. I don't
like the way above comment is worded even though it is helpful to
understand the calculation. If you want, then you can add such a
comment in file header, here it looks out of place.

2.
+power-of-2 groups, called "split points" in the code. That means on every new
+split points we double the existing number of buckets.

split points/split point

3.
+ * which phase of allocation the bucket_num belogs to with in the group.

/belogs/belongs

I have still not completely reviewed the patch as I have ran out of
time for today.

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