On 23.06.2013 01:48, Simon Riggs wrote:
On 22 June 2013 21:40, Stephen Frost<sfr...@snowman.net> wrote:
I'm actually not a huge fan of this as it's certainly not cheap to do. If it
can be shown to be better than an improved heuristic then perhaps it would
work but I'm not convinced.
We need two heuristics, it would seem:
* an initial heuristic to overestimate the number of buckets when we
have sufficient memory to do so
* a heuristic to determine whether it is cheaper to rebuild a dense
hash table into a better one.
Although I like Heikki's rebuild approach we can't do this every x2
overstretch. Given large underestimates exist we'll end up rehashing
5-12 times, which seems bad.
It's not very expensive. The hash values of all tuples have already been
calculated, so rebuilding just means moving the tuples to the right bins.
Better to let the hash table build and
then re-hash once, it we can see it will be useful.
That sounds even less expensive, though.
- Heikki
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