Re: Hash index todo list item

Kenneth Marshall wrote:
> I understand that a hash value is a many-to-one mapping. That is the
> point of the flag in the index. The flag means that there is only one
> item in the heap corresponding to that hash value. In this case we
> know that the value in the heap is the correct one and a possibly
> very expensive string comparison can be skipped. Given that the hash
> function is doing its job, almost every string comparison can be skipped.
> How long would it take to compare 1-32K of data? How much CPU usage?
> With this field in place, you only need to check tuple visibility
The value comparison cannot be skipped. I do not think you understand 
the many-to-one mapping in its entirety.

Example:
   Table has: a(1), b(2), c(3)   Index has: 1 => 1 (unique), 2 => 2 (unique), 3 => 3 (unique)

Query:
   select * from table where key = 'z';

If 'z' hashes to '3' (completely possible), then the index record 3 
points to tuple 3, and it "exists". Only, it doesn't because 'a' <> 'z'. 
You MUST check the value.

Cheers,
mark

-- 
Mark Mielke <mark@mielke.cc>