Re: Sequential scans

"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.

--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com