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I looked over the patch, and while it does fix the problem for SERIAL, I
am concerned about expecting users to user ::regclass in normal usage,
and I am concerned about adding something we will have to support in the
future when we come up with a better solution. Why is regclass not
being used automatically?

---------------------------------------------------------------------------

Tom Lane wrote:
> Attached is a fully-worked-out patch to make SERIAL column default
> expressions refer to the target sequence with a "regclass" literal
> instead of a "text" literal. Since the regclass literal is actually
> just an OID, it is impervious to renamings and schema changes of
> the target sequence. This fixes the long-standing hazard of renaming
> a serial column's sequence, as well as the recently added hazard of
> renaming the schema the sequence is in; and it lets us get rid of a
> very klugy solution in ALTER TABLE SET SCHEMA.
>
> I've arranged for stored regclass literals to create dependencies on
> the referenced relation, which provides useful improvements even for
> handwritten defaults: given
>
> create sequence myseq;
> create table foo (f1 int default nextval('myseq'::regclass));
>
> the system will not allow myseq to be dropped while the default
> expression remains. (This also ensures that pg_dump will emit the
> sequence before the table.)
>
> The patch also fixes a couple of places where code was still looking
> at the deprecated pg_attrdef.adsrc column, instead of reverse-compiling
> pg_attrdef.adbin. This ensures that psql's \d command shows the
> up-to-date form of a column default. (That should have happened quite
> some time ago; not sure why it was overlooked.)
>
> I propose applying this to fix the open issue that ALTER SCHEMA RENAME
> breaks serial columns. Comments, objections?
>
> regards, tom lane
>

Content-Description: seq-regclass.patch.gz

[ Type application/octet-stream treated as attachment, skipping... ]

>
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Bruce Momjian | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 359-1001
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Bruce Momjian <pgman@candle.pha.pa.us> writes:
> I looked over the patch, and while it does fix the problem for SERIAL, I
> am concerned about expecting users to user ::regclass in normal usage,
> and I am concerned about adding something we will have to support in the
> future when we come up with a better solution. Why is regclass not
> being used automatically?

If we provide both nextval(text) and nextval(regclass), then the parser
will interpret "nextval('something')" as nextval(text) because that's
the more preferred resolution of an unknown-type literal. The only way
to make regclass be used "automatically" would be to remove the
text-input variant. That is where I want to go eventually, but it seems
pretty risky to jump there in one step. The proposed patch adds
regclass-based functions alongside the existing functionality, so that
people can migrate as they choose; it does not open any risks of
breaking cases that work now.

regards, tom lane

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aTom Lane wrote:
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > I looked over the patch, and while it does fix the problem for SERIAL, I
> > am concerned about expecting users to user ::regclass in normal usage,
> > and I am concerned about adding something we will have to support in the
> > future when we come up with a better solution. Why is regclass not
> > being used automatically?
>
> If we provide both nextval(text) and nextval(regclass), then the parser
> will interpret "nextval('something')" as nextval(text) because that's
> the more preferred resolution of an unknown-type literal. The only way
> to make regclass be used "automatically" would be to remove the
> text-input variant. That is where I want to go eventually, but it seems
> pretty risky to jump there in one step. The proposed patch adds
> regclass-based functions alongside the existing functionality, so that
> people can migrate as they choose; it does not open any risks of
> breaking cases that work now.

What I am primarily worried about in your patch is the exposure of
::regclass as a recommended way of doing things. I know we can
discourage its us later, but once people start using something, it is
hard to change.

Right now, we have three cases, SERIAL, DEFAULT with no-schema seqname,
and DEFAULT with schema-specified seqname. If we just do regclass
internally for SERIAL, we don't have any user-visible change, except for
the psql \d display of the default.

The second case already works because there is no class name. It is
only the last one where recommending regclass helps, but is it worth
improving sequence/schema renaming by exposing and recommending a
::regclass syntax that will go away as soon as we fix this properly?

--
Bruce Momjian | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 359-1001
+ If your life is a hard drive, | 13 Roberts Road
+ Christ can be your backup. | Newtown Square, Pennsylvania 19073

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Bruce Momjian <pgman@candle.pha.pa.us> writes:
> What I am primarily worried about in your patch is the exposure of
> ::regclass as a recommended way of doing things. I know we can
> discourage its us later, but once people start using something, it is
> hard to change.

Why shouldn't it be a recommended way of doing things? It is certainly
far better than the existing text-argument way.

> Right now, we have three cases, SERIAL, DEFAULT with no-schema seqname,
> and DEFAULT with schema-specified seqname. If we just do regclass
> internally for SERIAL, we don't have any user-visible change, except for
> the psql \d display of the default.

> The second case already works because there is no class name.

No, it really wouldn't "work" at all. It's unsafe if the user changes
the search path for example, and it certainly doesn't handle any of the
renaming or change-of-schema cases.

> It is
> only the last one where recommending regclass helps, but is it worth
> improving sequence/schema renaming by exposing and recommending a
> ::regclass syntax that will go away as soon as we fix this properly?

Please explain what you think a "proper" fix is. I think this patch is
a proper fix. I see no better alternative that we might implement
later.

The only other thing that's been discussed is the SQL2003 syntax
NEXT VALUE FOR sequencename
but this is in fact just syntactic sugar for something functionally
equivalent to nextval('sequencename'::regclass). It cannot completely
replace all uses of the nextval function, because only a constant table
name can appear.

Hmm ... given the proposed patch, it would indeed take only a few more
lines in gram.y to support the NEXT VALUE FOR syntax ...

regards, tom lane

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I wrote:
> The only other thing that's been discussed is the SQL2003 syntax
> NEXT VALUE FOR sequencename
> but this is in fact just syntactic sugar for something functionally
> equivalent to nextval('sequencename'::regclass).

I have to take that back. It's not just syntactic sugar for nextval(),
because the SQL2003 spec says

: If there are multiple instances of <next value expression>s specifying
: the same sequence generator within a single SQL-statement, all those
: instances return the same value for a given row processed by that
: SQL-statement.

So it's really sort of a magic combination of nextval() and currval().
To meet the spec semantics, we'd need some sort of layer over nextval()
that would keep track of whether a new value should be obtained or not.

I don't think we should use the spec syntax until we're prepared to
meet the spec semantics, so NEXT VALUE FOR as part of the current patch
seems "out".

A relatively simple Plan B would be to use different SQL names for the
variant functions, ie, keep nextval() as is and instead invent, say,
next_value(regclass). Then we tell people to use next_value('foo')
and they don't need to write the cast explicitly. This seems
notationally nicer but a major pain in the neck from the point of view
of documentation and explanation --- for instance, instead of saying
"nextval does this" we'd have to say "next_value and nextval do this".
Not at all sure that I like it better.

regards, tom lane

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Tom Lane wrote:
> I wrote:
> > The only other thing that's been discussed is the SQL2003 syntax
> > NEXT VALUE FOR sequencename
> > but this is in fact just syntactic sugar for something functionally
> > equivalent to nextval('sequencename'::regclass).
>
> I have to take that back. It's not just syntactic sugar for nextval(),
> because the SQL2003 spec says
>
> : If there are multiple instances of <next value expression>s specifying
> : the same sequence generator within a single SQL-statement, all those
> : instances return the same value for a given row processed by that
> : SQL-statement.
>
> So it's really sort of a magic combination of nextval() and currval().
> To meet the spec semantics, we'd need some sort of layer over nextval()
> that would keep track of whether a new value should be obtained or not.
>
> I don't think we should use the spec syntax until we're prepared to
> meet the spec semantics, so NEXT VALUE FOR as part of the current patch
> seems "out".

OK.

> A relatively simple Plan B would be to use different SQL names for the
> variant functions, ie, keep nextval() as is and instead invent, say,
> next_value(regclass). Then we tell people to use next_value('foo')
> and they don't need to write the cast explicitly. This seems
> notationally nicer but a major pain in the neck from the point of view
> of documentation and explanation --- for instance, instead of saying
> "nextval does this" we'd have to say "next_value and nextval do this".
> Not at all sure that I like it better.

Agreed, two names is a mess.

I still think we shouldn't be hashing this out during beta, but ...

What would the final nextval() behavior be? ::regclass binding? How
would late binding be done? What syntax?

--
Bruce Momjian | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 359-1001
+ If your life is a hard drive, | 13 Roberts Road
+ Christ can be your backup. | Newtown Square, Pennsylvania 19073

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Bruce Momjian <pgman@candle.pha.pa.us> writes:
> I still think we shouldn't be hashing this out during beta, but ...

We're looking at ways to fix some bugs. It's never been the case that
our first-resort response to a bug is "pull out features".

> What would the final nextval() behavior be? ::regclass binding? How
> would late binding be done? What syntax?

If I were prepared to say all that today, I would have just done it ;-)

The more I think about it, the more I think that two sets of function
names might not be such an awful idea. next_value(), curr_value(), and
set_value() seem like they'd work well enough. Then we'd just say that
nextval and friends are deprecated except when you need late binding,
and we'd be done.

regards, tom lane

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Tom Lane wrote:

>The more I think about it, the more I think that two sets of function
>names might not be such an awful idea. next_value(), curr_value(), and
>set_value() seem like they'd work well enough. Then we'd just say that
>nextval and friends are deprecated except when you need late binding,
>and we'd be done.
>
>
>
>

Personally, I like this more than the overloading idea.

cheers

andrew

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On Wed, 28 Sep 2005, Tom Lane wrote:

> I wrote:
> > The only other thing that's been discussed is the SQL2003 syntax
> > NEXT VALUE FOR sequencename
> > but this is in fact just syntactic sugar for something functionally
> > equivalent to nextval('sequencename'::regclass).
>
> I have to take that back. It's not just syntactic sugar for nextval(),
> because the SQL2003 spec says
>
> : If there are multiple instances of <next value expression>s specifying
> : the same sequence generator within a single SQL-statement, all those
> : instances return the same value for a given row processed by that
> : SQL-statement.
>
> So it's really sort of a magic combination of nextval() and currval().
> To meet the spec semantics, we'd need some sort of layer over nextval()
> that would keep track of whether a new value should be obtained or not.
>
> I don't think we should use the spec syntax until we're prepared to
> meet the spec semantics, so NEXT VALUE FOR as part of the current patch
> seems "out".

Well, AFAICT, the only part of the spec we cannot implement is what you
quote above. Therefore, why can't we support NEXT VALUE FOR seqname and
reject table creation/alteration which would add more than one reference
to the same sequence. That will allow us to avoid an intermediate step
in getting to the SQL2003 syntax. Having to support three different
sequence incrementation mechanisms for three flavours of PostgreSQL is
going to be a real PITA.

Thanks,

Gavin

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Gavin Sherry wrote:
> > So it's really sort of a magic combination of nextval() and currval().
> > To meet the spec semantics, we'd need some sort of layer over nextval()
> > that would keep track of whether a new value should be obtained or not.
> >
> > I don't think we should use the spec syntax until we're prepared to
> > meet the spec semantics, so NEXT VALUE FOR as part of the current patch
> > seems "out".
>
> Well, AFAICT, the only part of the spec we cannot implement is what you
> quote above. Therefore, why can't we support NEXT VALUE FOR seqname and
> reject table creation/alteration which would add more than one reference
> to the same sequence. That will allow us to avoid an intermediate step
> in getting to the SQL2003 syntax. Having to support three different
> sequence incrementation mechanisms for three flavours of PostgreSQL is
> going to be a real PITA.

Well, that is an _excellent_ point. We would have three mechanisms,
which is confusing.

--
Bruce Momjian | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 359-1001
+ If your life is a hard drive, | 13 Roberts Road
+ Christ can be your backup. | Newtown Square, Pennsylvania 19073

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