Constraint documentation

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Hello lætitia,

My 0.02 € to try to simplify the suggested documentation.

Check constraint were not

are not

designed to enforce business rules across tables.

Avoid using check constraints with function accessing to other tables

accessing other tables (no "to")

and prefer triggers instead (please refer to <xref linkend="triggers"/>
for more information about triggers).

... and use <xref linkend="triggers"/> instead.

PostgreSQL won't prevent you from doing so,

Although PostgreSQL ... so,

but be aware you might encounter difficulties to restore dumps
(generated with pg_dump or pg_dumpall) if you do.

beware that dumps generated by <application>pg_*<...> or <...> may be hard
to restore because of such checks, as the underlying dependencies are not
taken into account.

--
Fabien.

Thanks!
I'll correct the patch ASAP including your modifications.

Le sam. 23 juin 2018 à 19:15, Fabien COELHO <coelho@cri.ensmp.fr> a écrit :

Hello lætitia,

My 0.02 € to try to simplify the suggested documentation.

Check constraint were not

are not

designed to enforce business rules across tables.

Avoid using check constraints with function accessing to other tables

accessing other tables (no "to")

and prefer triggers instead (please refer to <xref linkend="triggers"/>
for more information about triggers).

... and use <xref linkend="triggers"/> instead.

PostgreSQL won't prevent you from doing so,

Although PostgreSQL ... so,

but be aware you might encounter difficulties to restore dumps
(generated with pg_dump or pg_dumpall) if you do.

beware that dumps generated by <application>pg_*<...> or <...> may be hard
to restore because of such checks, as the underlying dependencies are not
taken into account.

--
Fabien.

--
*Think! Do you really need to print this email ? *
*There is no Planet B.*

Hello,

Ok, I corrected the patch as suggested. I hope I did it right.

Have a nice day,

Lætitia

Le lun. 25 juin 2018 à 16:02, Lætitia Avrot <laetitia.avrot@gmail.com> a
écrit :

Thanks!
I'll correct the patch ASAP including your modifications.

Le sam. 23 juin 2018 à 19:15, Fabien COELHO <coelho@cri.ensmp.fr> a
écrit :

Hello lætitia,

My 0.02 € to try to simplify the suggested documentation.

Check constraint were not

are not

designed to enforce business rules across tables.

Avoid using check constraints with function accessing to other tables

accessing other tables (no "to")

and prefer triggers instead (please refer to <xref linkend="triggers"/>
for more information about triggers).

... and use <xref linkend="triggers"/> instead.

PostgreSQL won't prevent you from doing so,

Although PostgreSQL ... so,

but be aware you might encounter difficulties to restore dumps
(generated with pg_dump or pg_dumpall) if you do.

beware that dumps generated by <application>pg_*<...> or <...> may be
hard
to restore because of such checks, as the underlying dependencies are not
taken into account.

--
Fabien.

--
*Think! Do you really need to print this email ? *
*There is no Planet B.*

--
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*There is no Planet B.*

On 25/06/18 17:45, Lætitia Avrot wrote:

+   <note>
+    <para>
+     Check constraint are not designed to enforce business rules across tables.
+     Avoid using check constraints with function accessing other tables and

"with functions" or "with a function". I prefer the former.

+     use <xref linkend="triggers"/> instead. Although PostgreSQL won't prevent you
+     from doing so, but beware that dumps generated by <application>pg_dump</application>

No but.

+     or <application>pg_dumpall</application> may be hard
+     to restore because of such checks, as the underlying dependencies are not
+     taken into account.
+    </para>
+   </note>

--
Vik Fearing +33 6 46 75 15 36
http://2ndQuadrant.fr PostgreSQL : Expertise, Formation et Support

On 25/06/18 17:45, Lætitia Avrot wrote:

+   <note>
+    <para>
+     Check constraint are not designed to enforce business rules across

tables.

+ Avoid using check constraints with function accessing other tables

and

Subject/verb agreement - either "A check constraint is ..." or "Check
constraints are ..." would be appropriate.

Hi,

Thanks a lot. I did the modification. It's in the patch enclosed.

Have a nice day,

Lætitia

Le mar. 26 juin 2018 à 01:42, Brad DeJong <bpd0018@gmail.com> a écrit :

On 25/06/18 17:45, Lætitia Avrot wrote:

+   <note>
+    <para>
+     Check constraint are not designed to enforce business rules across

tables.

+ Avoid using check constraints with function accessing other tables

and

Subject/verb agreement - either "A check constraint is ..." or "Check
constraints are ..." would be appropriate.

--
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*There is no Planet B.*

On 26/06/18 09:49, Lætitia Avrot wrote:

Thanks a lot. I did the modification. It's in the patch enclosed.

Looks good to me.
--
Vik Fearing +33 6 46 75 15 36
http://2ndQuadrant.fr PostgreSQL : Expertise, Formation et Support

On Tue, Jun 26, 2018 at 4:24 AM, Vik Fearing <vik.fearing@2ndquadrant.com>
wrote:

Looks good to me.

I'll second that. Looks good to me too.

On 6/26/18 09:49, Lætitia Avrot wrote:

+   <note>
+    <para>
+     Check constraints are not designed to enforce business rules across tables.
+     Avoid using check constraints with a function accessing other tables and
+     use <xref linkend="triggers"/> instead. Although PostgreSQL won't prevent you
+     from doing so, beware that dumps generated by <application>pg_dump</application>
+     or <application>pg_dumpall</application> may be hard
+     to restore because of such checks, as the underlying dependencies are not
+     taken into account.
+    </para>
+   </note>

In a way, I think this is attacking the wrong problem. It is saying
that you should use triggers instead of check constraints in certain
circumstances. But triggers are also used as an implementation detail
of constraints. While it currently doesn't exist, a deferrable check
constraint would probably be implemented as a trigger. It's not the
triggerness that fixes this problem. The problem is more generally that
if a function uses a table, then pg_dump can't know about the ordering.
It happens to work for triggers because triggers are dumped after all
tables, as a performance optimization, and we could very well dump check
constraints differently as well.

--
Peter Eisentraut http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Hello Peter,

+   <note>
+    <para>
+     Check constraints are not designed to enforce business rules across tables.
+     Avoid using check constraints with a function accessing other tables and
+     use <xref linkend="triggers"/> instead. Although PostgreSQL won't prevent you
+     from doing so, beware that dumps generated by <application>pg_dump</application>
+     or <application>pg_dumpall</application> may be hard
+     to restore because of such checks, as the underlying dependencies are not
+     taken into account.
+    </para>
+   </note>

In a way, I think this is attacking the wrong problem. It is saying
that you should use triggers instead of check constraints in certain
circumstances.

Yep. The circumstance is being able to dump & restore the database. If you
do not care about dump/restore, check constraints are always fine.

But triggers are also used as an implementation detail of constraints.

Yep.

It's not the triggerness that fixes this problem. The problem is more
generally that if a function uses a table, then pg_dump can't know about
the ordering.

Yep.

It happens to work for triggers because triggers are dumped after all
tables, as a performance optimization, and we could very well dump check
constraints differently as well.

Sure.

ISTM that is more or less what the text is saying?

I'm not sure what is the suggestion wrt to the documentation text. Is the
issue only with the first introductory sentence? Would removing it be
enough?

--
Fabien.

On 07.07.18 10:23, Fabien COELHO wrote:

I'm not sure what is the suggestion wrt to the documentation text. Is the
issue only with the first introductory sentence? Would removing it be
enough?

Yes. But it would be even better to fix pg_dump.

--
Peter Eisentraut http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Hello Peter,

I'm not sure what is the suggestion wrt to the documentation text. Is the
issue only with the first introductory sentence? Would removing it be
enough?

Yes. But it would be even better to fix pg_dump.

Sure. The purpose of Lætitia patch is simply to document the consequences
if the current behavior. Fixing pg_dump is another issue:-)

I guess that this would involve postponing all non trivial CHECK
declarations to after all table and function creations. While waiting for
such a significant change, ISTM that documenting the issue is a reasonable
option.

--
Fabien.

Hi Peter,

I understand what you're pointing at and I agree that it could be a good
thing to be able to dump/restore a table without problem.

My point was that check constraints weren't supposed to be used that way
theorically (or maybe i'm mistaken ?) so I thought maybe we should just
inform the user that this kind of use of a check constraint is a misuse of
that feature.

Maybe it's not the right way to say it. I can remove the part about pg_dump
if it's too confusing...

Regards,

Lætitia

Le mer. 27 juin 2018 à 08:44, Peter Eisentraut <
peter.eisentraut@2ndquadrant.com> a écrit :

On 6/26/18 09:49, Lætitia Avrot wrote:

+   <note>
+    <para>
+     Check constraints are not designed to enforce business rules

across tables.

+ Avoid using check constraints with a function accessing other

tables and

+ use <xref linkend="triggers"/> instead. Although PostgreSQL won't

prevent you

+ from doing so, beware that dumps generated by

<application>pg_dump</application>

+     or <application>pg_dumpall</application> may be hard
+     to restore because of such checks, as the underlying dependencies

are not

+     taken into account.
+    </para>
+   </note>

In a way, I think this is attacking the wrong problem. It is saying
that you should use triggers instead of check constraints in certain
circumstances. But triggers are also used as an implementation detail
of constraints. While it currently doesn't exist, a deferrable check
constraint would probably be implemented as a trigger. It's not the
triggerness that fixes this problem. The problem is more generally that
if a function uses a table, then pg_dump can't know about the ordering.
It happens to work for triggers because triggers are dumped after all
tables, as a performance optimization, and we could very well dump check
constraints differently as well.

--
Peter Eisentraut http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

--
*Think! Do you really need to print this email ? *
*There is no Planet B.*

On 2018-Aug-07, L�titia Avrot wrote:

Hi Peter,

I understand what you're pointing at and I agree that it could be a good
thing to be able to dump/restore a table without problem.

My point was that check constraints weren't supposed to be used that way
theorically (or maybe i'm mistaken ?) so I thought maybe we should just
inform the user that this kind of use of a check constraint is a misuse of
that feature.

Tom Lane pointed out in another thread that the SQL standard lists
feature F673 "Reads SQL-data routine invocations in CHECK constraints"
which permits CHECK constraints to examine tables, so saying "you're not
supposed to do this", while correct from a Postgres perspective, would
be short-sighted ISTM, because we will make ourselves liars as soon as
we implement the feature.

I agree that we should point this out in *some* way, just not sure how.
Maybe something like "Postgres does not currently support CHECK
constraints containing queries, therefore we recommend to avoid them."
I would not mention pg_dump by name, just say dumps may not restore
depending on phase of moon.

(BTW I'm not sure of the term "other tables". You could have a query
that references the same table ...)

--
�lvaro Herrera https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

On Thu, Aug 9, 2018 at 10:32 PM, Alvaro Herrera <alvherre@2ndquadrant.com>
wrote:

On 2018-Aug-07, Lætitia Avrot wrote:

Hi Peter,

I understand what you're pointing at and I agree that it could be a good
thing to be able to dump/restore a table without problem.

My point was that check constraints weren't supposed to be used that way
theorically (or maybe i'm mistaken ?) so I thought maybe we should just
inform the user that this kind of use of a check constraint is a misuse

of

that feature.

Tom Lane pointed out in another thread that the SQL standard lists
feature F673 "Reads SQL-data routine invocations in CHECK constraints"
which permits CHECK constraints to examine tables, so saying "you're not
supposed to do this", while correct from a Postgres perspective, would
be short-sighted ISTM, because we will make ourselves liars as soon as
we implement the feature.

I agree that we should point this out in *some* way, just not sure how.
Maybe something like "Postgres does not currently support CHECK
constraints containing queries, therefore we recommend to avoid them."
I would not mention pg_dump by name, just say dumps may not restore
depending on phase of moon.

(BTW I'm not sure of the term "other tables". You could have a query
that references the same table ...)

--
Álvaro Herrera https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

I like this:

"Postgres does not currently support CHECK constraints containing

queries, therefore we recommend to avoid them."

Perhaps adding:

CHECK constraints are currently meant to be used as *row constraints*

only.

Use - if possible - UNIQUE or EXCLUDE constraints. for constraints that

involve many or all rows of a table,

and FOREIGN KEY constraints for cross table constraints.
More complex constraints will be available when ASSERTION are implemented.

And then adding some warning about using functions in CHECK constraints to
bypass current limitations.

Pantelis Theodsoiou

On 09/08/2018 23:32, Alvaro Herrera wrote:

I agree that we should point this out in *some* way, just not sure how.
Maybe something like "Postgres does not currently support CHECK
constraints containing queries, therefore we recommend to avoid them."
I would not mention pg_dump by name, just say dumps may not restore
depending on phase of moon.

I think it would be very easy to restore check constraints separately
after all tables in pg_dump. There is already support for that, but
it's only used when necessary, for things like not-valid constraints.
The argument in favor of keeping the constraint with the table is
probably only aesthetics, but of course the argument against is that it
sometimes doesn't work. So we could either enhance the smarts about
when to use the "dump separately" path (this might be difficult), or
just use it always.

--
Peter Eisentraut http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Peter Eisentraut <peter.eisentraut@2ndquadrant.com> writes:

I think it would be very easy to restore check constraints separately
after all tables in pg_dump. There is already support for that, but
it's only used when necessary, for things like not-valid constraints.
The argument in favor of keeping the constraint with the table is
probably only aesthetics,

No, it's mainly about performance. Checking the constraint at data load
time avoids extra scans of the table, and should work in any case that
we consider supported.

To be clear, I totally reject the notion that we should consider this
case supported, or that kluging pg_dump to not fail would make it so.
As a counterexample, if you have a poor-mans-FK check constraint on
table A that only succeeds when there's a matching row in table B, it
cannot prevent the case where you insert a valid (matching) row in
table A and then later delete its matching row in B.

Maybe someday we'll have full database assertions (with, no doubt,
a ton of performance caveats). In the meantime, let's not slow down
CHECK constraints for everyone in order to partially fix a
fundamentally broken use-case. If the documentation isn't clear enough
about such cases being unsupported, by all means let's make it so.

regards, tom lane

On Fri, Aug 10, 2018 at 12:27:49PM +0200, Peter Eisentraut wrote:

On 09/08/2018 23:32, Alvaro Herrera wrote:

I agree that we should point this out in *some* way, just not sure how.
Maybe something like "Postgres does not currently support CHECK
constraints containing queries, therefore we recommend to avoid them."
I would not mention pg_dump by name, just say dumps may not restore
depending on phase of moon.

I think it would be very easy to restore check constraints separately
after all tables in pg_dump. There is already support for that, but
it's only used when necessary, for things like not-valid constraints.
The argument in favor of keeping the constraint with the table is
probably only aesthetics, but of course the argument against is that it
sometimes doesn't work. So we could either enhance the smarts about
when to use the "dump separately" path (this might be difficult), or
just use it always.

+1 for dumping all constraints separately by default.

Currently, it's possible to create unrestorable databases without
fiddling with the catalog, as a legacy database I was dealing with
just last week demonstrated.

It occurs to me that the aesthetic issues might be dealt with by
having a separate "aesthetic" restore mode, which is to say what you'd
expect if you were writing the schema code /de novo/. This would be
non-trivial to get right in some cases, and flat-out impossible for
cases where we can't see into the code that could produce a
dependency.

Best,
David.
--
David Fetter <david(at)fetter(dot)org> http://fetter.org/
Phone: +1 415 235 3778

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