Re: POC: converting Lists into arrays

On Sat, Feb 23, 2019 at 9:24 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Every time this has come up, I've opined that the right fix is to jack
> up the List API and drive a new implementation underneath, as we did
> once before (cf commit d0b4399d81). I thought maybe it was about time
> to provide some evidence for that position, so attached is a POC patch
> that changes Lists into expansible arrays, while preserving most of
> their existing API.

I'm not really convinced that this is the way to go. The thing is,
any third-party code people have that uses a List may simply break.
If you kept the existing List and changed a bunch of existing code to
use a new Vector implementation, or Thomas's SimpleVector stuff, then
that wouldn't happen. The reason why people - or at least me - have
been reluctant to accept that you can just jack up the API and drive a
new implementation underneath is that the new implementation will
involve breaking guarantees on which existing code relies; indeed,
your email makes it pretty clear that this is the case. If you could
replace the existing implementation without breaking any code, that
would be a no-brainer but there's no real way to do that and get the
performance benefits you're seeking to obtain.

It is also perhaps worth mentioning that reimplementing a List as an
array means that it is... not a list. That is a pretty odd state of
affairs, and to me is another sign that we want to leave the existing
thing alone and convert some/most/all core code to use a new thing.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
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