Re: Sequential scans

Gregory Stark wrote:
> "Heikki Linnakangas" <heikki(at)enterprisedb(dot)com> writes:
>
>> Let's use a normal hash table instead, and use a lock to protect it. If we only
>> update it every 10 pages or so, the overhead should be negligible. To further
>> reduce contention, we could modify ReadBuffer to let the caller know if the
>> read resulted in a physical read or not, and only update the entry when a page
>> is physically read in. That way all the synchronized scanners wouldn't be
>> updating the same value, just the one performing the I/O. And while we're at
>> it, let's use the full relfilenode instead of just the table oid in the hash.
>
> It's probably fine to just do that. But if we find it's a performance
> bottleneck we could probably still manage to avoid the lock except when
> actually inserting a new hash element. If you just store in the hash an index
> into an array stored in global memory then you could get away without a lock
> on the element in the array.
>
> It starts to get to be a fair amount of code when you think about how you
> would reuse elements of the array. That's why I suggest only looking at this
> if down the road we find that it's a bottleneck.

Another trick you could do is to use acquire the lock conditionally when
updating it. But I doubt it's a problem anyhow, if we put some sane
lower limit in there so that it's not used at all for small tables.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com