Kenneth Marshall wrote:
On Sun, Sep 02, 2007 at 10:41:22PM -0400, Tom Lane wrote:
Kenneth Marshall <[EMAIL PROTECTED]> writes:
... This is the rough plan. Does anyone see anything critical that
is missing at this point?
Sounds pretty good.  Let me brain-dump one item on you: one thing that
hash currently has over btree is the ability to handle index items up
to a full page.  Now, if you go with a scheme that only stores hash
codes and not the underlying data, you can not only handle that but
improve on it; but if you reduce the bucket size and don't remove the
data, it'd be a step backward.  The idea I had about dealing with that
was to only reduce the size of primary buckets --- if it's necessary to
add overflow space to a bucket, the overflow units are still full pages.
So an index tuple up to a page in size can always be accommodated by
adding an overflow page to the bucket.

Just a thought, but AFAIR it's not in the archives anywhere.

                        regards, tom lane

I was thinking about this some more, and it strikes me that we can
keep the page size = bucket size = overflow size in the new scheme
of storing the hash value in the index and still reduce the effective
bucket size. Let's make the new size the (page size / 2^n) where n
is chosen appropriately. Then we we store the value in the page we
simply use n bits of the hash to determine which sub-piece of the
page to use to actually store the value. We may need to add a multiplier
to adjust the decision to split the page based on the mini-page. This
should allow us to much more densely pack the pages and approach the
1 item per bucket. This could easily shrink the size of the index by
a factor of 2^n. Let me know what you think.
My personal opinion is that something like this is required to take best advantage of hashes. I didn't respond immediately because I don't have advice on what the best implementation would look like.

I have also been wondering about the effect of a hash index that includes multiple values to the same key (either a non-unique key, or different tuples from the same key due to table updates). I started by thinking that the maximum number of hash entries per hash bucket should be kept to 2 or 3 to prevent reduction in performance to that of btree, but I think this doesn't work if multiple tuples can have the same key. Unless - the maps is hash value =1:1> index page =1:1> hash bucket =1:N> hash value =1:M=> tuples. Guarantee than N is small (either <= 2 or <=4 depending on performance evaluation) by re-hashing if N ever becomes > 2 or > 4. Choose a sparse harsh. Let 1:M be indefinite? Also, optimize the 1:M=1:1 case, such that the value can be inline?

For most cases, I would think the above model would make it cheap to determine if the key does not exist, as well as the 1:1 (hash value:key) case requiring a single page lookup. Update performance would suffer, but lookup should be faster than btree in these cases, as btree often requires > 1 index page scan.



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