Re: [PATCHES] Proposed patch for sequence-renaming problems
Tom Lane wrote:
Here's an updated version of the patch. There's now just one nextval()
function, taking regclass, and backwards compatibility is handled
through an implicit text-to-regclass cast. Existing dumps will not see
any behavioral changes because nextval('foo') will be dumped as
nextval('foo'::text), but new entries of nextval('foo') will be
captured as regclass constants instead.
I noted that this version caused a couple more regression tests to fail;
for instance, the constraints test was expecting that it could drop and
recreate a sequence that was referenced by a default expression spelled
as nextval('foo'). So we are paying for improved ease of use by
taking a larger backwards-compatibility risk than the original patch
did.
Last call for objections ...
No objection, but +1 from me. If this is the best solution people can
agree on, better now than later. The missing dependencies for sequences
were a bug in the first place, IMHO.
Best Regards,
Michael Paesold
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Re: [PATCHES] Proposed patch for sequence-renaming problems
Gavin Sherry [EMAIL PROTECTED] writes: 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. And how are you going to determine whether a query (not a table definition) contains more than one NEXT VALUE FOR the same sequence? Bear in mind some of them could be hidden down inside views or functions. regards, tom lane ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [PATCHES] Proposed patch for sequence-renaming problems
Here's an updated version of the patch. There's now just one nextval()
function, taking regclass, and backwards compatibility is handled
through an implicit text-to-regclass cast. Existing dumps will not see
any behavioral changes because nextval('foo') will be dumped as
nextval('foo'::text), but new entries of nextval('foo') will be
captured as regclass constants instead.
I noted that this version caused a couple more regression tests to fail;
for instance, the constraints test was expecting that it could drop and
recreate a sequence that was referenced by a default expression spelled
as nextval('foo'). So we are paying for improved ease of use by
taking a larger backwards-compatibility risk than the original patch
did.
Last call for objections ...
regards, tom lane
binTLINvTHZEG.bin
Description: seq-regclass-2.patch.gz
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Re: [PATCHES] Proposed patch for sequence-renaming problems
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 expressions 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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Re: [PATCHES] Proposed patch for sequence-renaming problems
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 expressions 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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Re: [PATCHES] Proposed patch for sequence-renaming problems
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 ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [PATCHES] Proposed patch for sequence-renaming problems
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 ---(end of broadcast)--- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
Re: [PATCHES] Proposed patch for sequence-renaming problems
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 expressions 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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Re: [PATCHES] Proposed patch for sequence-renaming problems
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 ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
Re: [PATCHES] Proposed patch for sequence-renaming problems
Gavin Sherry wrote: 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. Oh, if we went in that direction, how would we fix loading previous dumps? Isn't NEXT VALUE for going to map internally to the early-binding version of nextval(). I am thinking mucking with nextval() and its casts is going to be required for ALTER SCHEMA RENAME to work with any kind of reliability. -- 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 ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
[PATCHES] Proposed patch for sequence-renaming problems
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
bineyzK29dAnv.bin
Description: seq-regclass.patch.gz
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Re: [PATCHES] Proposed patch for sequence-renaming problems
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
+ If your life is a hard drive, | 13 Roberts Road
+ Christ can be your backup.| Newtown Square, Pennsylvania 19073
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Re: [PATCHES] Proposed patch for sequence-renaming problems
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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Re: [PATCHES] Proposed patch for sequence-renaming problems
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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