Re: BUG #15344: pg_proc.proisagg was removed incompatibly in PostgreSQL 11

On Tue, Aug 21, 2018 at 4:45 PM Andres Freund <andres(at)anarazel(dot)de> wrote:

> On 2018-08-21 16:39:18 +0200, Lukas Eder wrote:
> > On Tue, Aug 21, 2018 at 4:28 PM Andres Freund <andres(at)anarazel(dot)de>
> wrote:
> >
> > > Hi,
> > >
> > > On 2018-08-21 14:23:45 +0000, PG Bug reporting form wrote:
> > > > When comparing the current version (10) [1] and the developer version
> > > (11)
> > > > [2] of the pg_proc documentation, then it can be seen that the
> > > > pg_proc.proisagg column was removed backwards incompatibly. The
> > > > documentation states for [1]:
> > >
> > > Please note that the pg_catalog.* tables (and views) are *NOT* intended
> > > to backwards compatible between major versions. We change them in ways
> > > backward incompatible all the time.
> > >
> >
> > The pg_catalog tables do seem to be the only way to reverse engineer some
> > more sophisticated things in the database.
>
> Yes, there's some things that aren't represented in a standardized way
> in information_schema.*. If that's the case it's good for tool vendors
> to pipe up and ask for something intended to be externally visible.
>

I have mixed feelings about this. The information_schema is part of the SQL
standard. I reckon that this particular information would belong in
INFORMATION_SCHEMA.ROUTINES.ROUTINE_TYPE. The SQL:2011 standard (I don't
have newer versions of the SQL/Schemata document) mentions these possible
values for that column:

'PROCEDURE', 'FUNCTION', 'INSTANCE METHOD', 'STATIC METHOD', 'CONSTRUCTOR
METHOD'

So, no aggregate functions or window functions as in pg_proc.prokind. When
extending the standard, the functionality becomes a bit less standard, and
risks breaking as well in the future, e.g. when the standard *does* add
aggregate functions as a possible value, but not using the name PostgreSQL
chooses now.

Which is why the vendor specific pg_catalog is so useful. Any value is
acceptable in those tables as you do not have to coordinate their layout
with the standard committee.

In Oracle, the dictionary views aren't following the information_schema
standard, but are vastly richer than what the standard supports - just like
pg_catalog. They are definitely kept backwards compatible for the same
reason I've mentioned: Tool support.

A database product thrives on the quality of the tools supporting it.
Making it hard for the tool vendors might mean there's less support for
advanced features. From a market adoption perspective, in the long run,
there is no option but to be more backwards compatible.

> > I imagine that this is being
> > done by tool vendors like myself (jOOQ) quite a bit. And there are tons
> of
> > Stack Overflow answers that show how to query the pg_catalog tables, all
> of
> > them risking to be outdated between major versions.
>
> People doing bad things on stackoverflow isn't very convincing.... ;)
>

Your perception of "bad" is biased of course, just like mine. People use
what's available, this has always been the case with any product. If there
were "internal" and "public" catalog tables / views, then it would be more
understandable that using the (internal) pg_catalog is being dismissed, but
given that there is no option...

Here, have a quick google search for questions on Stack Overflow involving
pg_proc.proisagg:
https://www.google.ch/search?q=site%3Astackoverflow.com+proisagg

It returns 107 results on Stack Overflow alone. Some examples:

- https://stackoverflow.com/a/50093399/521799
- https://stackoverflow.com/a/18200250/521799
- https://stackoverflow.com/a/20549944/521799
- https://stackoverflow.com/a/24774064/521799
- https://stackoverflow.com/a/24034609/521799
- https://dba.stackexchange.com/a/46996/2512
- https://stackoverflow.com/a/48709779/521799
- https://stackoverflow.com/a/48709779/521799
- https://stackoverflow.com/a/308500/521799
- https://stackoverflow.com/a/20549944/521799

All of these answers are now outdated with PostgreSQL 11. Not only are they
outdated, but if the answers were fixed for PostgreSQL 11, the fixed
version wouldn't work on older PostgreSQL versions, because pg_proc.prokind
didn't exist earlier.

I find that a relatively high price to pay in this case for the relatively
easy solution to keep a pg_proc.proisagg computed column around for
backwards compatibility.

> > I understand that backwards compatibility is quite a bit of extra work,
> but
> > in cases like this particular one, the price to pay seems relatively low.
> > Perhaps a new strategy could be to break things only if there is really
> no
> > other solution?
>
> I mean we don't break things willy-nilly already. And there's plenty
> cases where we kept things around for backward compatibility. But
> usually the problem is that that means you have to keep the shim around
> forever. Even if better solutions have been around for many years,
> there'll be complaints about removing them. So IMO the backward compat
> price for pg_catalog.* has to be low enough that there's essentially no
> point in keeping it around forever, or so painful that it'd cost a lot
> of people a lot.
>

I agree that at some point, the price to pay is too high. But in this case,
a boolean value has been replaced by an enumeration, so it would be really
easy to maintain the boolean value as a computed column, right?