Re: Let's drop two obsolete features which are bear-traps for novices

All,

While there's argument about hash indexes, it looks like nobody minds if
the MONEY type goes bye-bye.  So, time for a patch ...

In the case of hash indexes, because we still have to have the hash
opclasses in core, there's no way that it could be pushed out as an
extension module even if we otherwise had full support for AMs as
extensions.  So what I hear you proposing is "let's break this so
thoroughly that it *can't* be fixed".  I'm not on board with that.
I think the WARNING will do just fine to discourage novices who are
not familiar with the state of the hash AM.  In the meantime, we
could push forward with the idea of making hash indexes automatically
unlogged, so that recovering from a crash wouldn't be quite so messy/
dangerous.

On 2014-11-01 10:18:03 -0700, Josh Berkus wrote:
> On 10/31/2014 03:07 PM, Tom Lane wrote:
> > I don't care one way or the other about the money type, but I will defend
> > hash indexes, especially seeing that we've already added a pretty
> > in-your-face warning as of 9.5:
> >
> > regression=# create table foo(f1 int);
> > CREATE TABLE
> > regression=# create index on foo using hash (f1);
> > WARNING:  hash indexes are not WAL-logged and their use is discouraged
> > CREATE INDEX
>
> Yes, and I'm arguing that is the wrong decision.  If hash indexes are
> "discouraged", then they shouldn't be in core in the first place.

Last time we discussed it there were people (IIRC Andrew was one of
them) commenting that they use hash indexes *precisely* because they're
not WAL logged and that they can live with the dangers that creates. I
don't think that's sufficient justification for introducing the feature
at all. But it's nothing new that removing a feature has to fit quite
different criteria than adding one.

So, by that argument we could remove hash indexes once we have unlogged
indexes on logged tables. But then there's no need to remove them
anymore...

I don't know if/when that will happen as such, but Simon was making noises
about writing code to treat hash indexes as unlogged automatically, which
would more or less fix the worst risks.  That's not just a special case
for hash indexes, but any index AM that lacks WAL support, as third-party
AMs might well do.