Re: [HACKERS] [PATCHES] Foreign key type checking patch
Hello again, I turn the discussion to the dev list as it seems more appropriate. So about the proposed patch to warn if foreign key type do not match the target key: Stephan Szabo [EMAIL PROTECTED] writes: I'm really not sure that it makes sense to warn for the fk cases where the semantics should be correct (if they're not we need to fix it or make it an error) but in which an error might have been made by the user because the types are different given that it at least seems reasonable to me that the fk type is allowable to be a subset of the referenced type. I don't think simply different types is sufficient to be warning material. I can think of several cases where it might be reasonable for the types to be different. One case in particular that needs some thought is where the FK and referenced PK are domains on a common base type. I'm looking forward to see an example where: 1) the difference in type is actually needed by the application. 2) a simple warning about the issue would be considered harmful. Let me describe some examples where IMVVHO a simple warning make sense, although they are silently accepted by postgres at the moment: 1/ integers CREATE TABLE foo(fid INT4 NOT NULL PRIMARY KEY, ...); CREATE TABLE bla(fid INT2 REFERENCES foo, ...); The application will be fine till you enter fid=32767, and it inserts will fail in bla with fid=32768. Much later on. 2/ chars CREATE TABLE foo(fid VARCHAR(4) NOT NULL PRIMARY KEY, ...); CREATE TABLE bla(fid VARCHAR(2) REFERENCES foo, ...); bla will be able to reference all 2-letters keys of foo, but no more. If you have some counter in foo, it will fail when it turns 3 letters. 3/ chars CREATE TABLE foo(fid VARCHAR(4) NOT NULL PRIMARY KEY, ...); CREATE TABLE bla(fid VARCHAR(8) REFERENCES foo, ...); declaring a larger size is not a problem here, however you will never be able to but any reference in bla larger than 4 as it must match its counter part in foo. So it is just a little bit stupid. 4/ time CREATE TABLE day(quand DATE NOT NULL PRIMARY KEY, ...); CREATE TABLE event(quand TIMESTAMP REFERENCES day, ...); The intent could be that events should refer to some day already registered in the base. Obviously it does work, because the = will cast to timestamp, to only the 00:00:00 timestamp will match a day. etc. -- Fabien Coelho - [EMAIL PROTECTED] ---(end of broadcast)--- TIP 7: don't forget to increase your free space map settings
Re: [HACKERS] [PATCHES] Foreign key type checking patch
On Tue, 2 Mar 2004, Fabien COELHO wrote: Hello again, I turn the discussion to the dev list as it seems more appropriate. So about the proposed patch to warn if foreign key type do not match the target key: Stephan Szabo [EMAIL PROTECTED] writes: I'm really not sure that it makes sense to warn for the fk cases where the semantics should be correct (if they're not we need to fix it or make it an error) but in which an error might have been made by the user because the types are different given that it at least seems reasonable to me that the fk type is allowable to be a subset of the referenced type. I don't think simply different types is sufficient to be warning material. I can think of several cases where it might be reasonable for the types to be different. One case in particular that needs some thought is where the FK and referenced PK are domains on a common base type. I'm looking forward to see an example where: 1) the difference in type is actually needed by the application. 2) a simple warning about the issue would be considered harmful. Let me describe some examples where IMVVHO a simple warning make sense, although they are silently accepted by postgres at the moment: 1/ integers CREATE TABLE foo(fid INT4 NOT NULL PRIMARY KEY, ...); CREATE TABLE bla(fid INT2 REFERENCES foo, ...); The application will be fine till you enter fid=32767, and it inserts will fail in bla with fid=32768. Much later on. Which is fine if bla is meant to store a subset of the allowable foo values. It'd be really hard to say at bla creation time that there isn't going to be a bla2 which say takes an int4 check (fid=32768) which might be being used for dividing up the foo space between multiple tables. 2/ chars CREATE TABLE foo(fid VARCHAR(4) NOT NULL PRIMARY KEY, ...); CREATE TABLE bla(fid VARCHAR(2) REFERENCES foo, ...); bla will be able to reference all 2-letters keys of foo, but no more. If you have some counter in foo, it will fail when it turns 3 letters. Same as above. 3/ chars CREATE TABLE foo(fid VARCHAR(4) NOT NULL PRIMARY KEY, ...); CREATE TABLE bla(fid VARCHAR(8) REFERENCES foo, ...); declaring a larger size is not a problem here, however you will never be able to but any reference in bla larger than 4 as it must match its counter part in foo. So it is just a little bit stupid. This one is fairly pointless for the single column case but a multiple column match unspecified constraint could allow the full 8 characters if there's a second column which is null. 4/ time CREATE TABLE day(quand DATE NOT NULL PRIMARY KEY, ...); CREATE TABLE event(quand TIMESTAMP REFERENCES day, ...); The intent could be that events should refer to some day already registered in the base. Obviously it does work, because the = will cast to timestamp, to only the 00:00:00 timestamp will match a day. This one does seem fairly broken. 5/ domains CREATE DOMAIN posint AS int4 check(value0); CREATE TABLE foo(fid int4 primary key); CREATE TABLE bla(fid posint references foo); The intent here is that foo may contain negative numbers but that those rows won't be referenced by bla. This is similar to 1 and 2. ---(end of broadcast)--- TIP 9: the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
Re: [HACKERS] [PATCHES] Foreign key type checking patch
Hello Stephan, CREATE TABLE foo(fid INT4 NOT NULL PRIMARY KEY, ...); CREATE TABLE bla(fid INT2 REFERENCES foo, ...); The application will be fine till you enter fid=32767, and it inserts will fail in bla with fid=32768. Much later on. Which is fine if bla is meant to store a subset of the allowable foo values. [...] Sure. This is NOT my point. I totally agree with you that the above example MAY BE what the user intends, and that it must be allowed. However it may ALSO be a bug that will pop up later on. Although it is POSSIBLE that this is fine, it is much more PROBABLE that it is a bug, hence I just suggest to issue a mere simple basic plain user-friendly little warning, what is quite different from issuing an error. Thus, the user has the information. He may chose to go on as that is what was meant, or maybe check the stuff and correct it. In postgres compilation, gcc uses the -Wall option to issue warnings about correct C constructs that may hide application bugs. This is the philosophy I'm suggesting here for this very small feature. Dear user, what you ask is right, however it looks a little bit strange, so I tell you just in case. I'm sure you're pretty happy that the gcc developers put such features for basic programmers, and that you use them;-) Why not allowing that kind of approach in postgres? CREATE TABLE foo(fid VARCHAR(4) NOT NULL PRIMARY KEY, ...); CREATE TABLE bla(fid VARCHAR(8) REFERENCES foo, ...); declaring a larger size is not a problem here, however you will never be able to but any reference in bla larger than 4 as it must match its counter part in foo. So it is just a little bit stupid. This one is fairly pointless Isn't it what I'm saying? for the single column case but a multiple column match unspecified constraint could allow the full 8 characters if there's a second column which is null. I do not understand. I can't see how you can put 8 characters in a reference which must match a 4 characters string. CREATE TABLE day(quand DATE NOT NULL PRIMARY KEY, ...); CREATE TABLE event(quand TIMESTAMP REFERENCES day, ...); The intent could be that events should refer to some day already registered in the base. Obviously it does work, because the = will cast to timestamp, to only the 00:00:00 timestamp will match a day. This one does seem fairly broken. Yes, my comment is broken, as I wrote it does work, meaning, it does not work:-( Anyway postgres accepts the two tables and the foreign key references from a timestamp to a date without a warning, and although I can imagine an application that would really mean that, I'm pretty sure most users would like this reported as a warning, as it is more likely to be a bug. CREATE DOMAIN posint AS int4 check(value0); CREATE TABLE foo(fid int4 primary key); CREATE TABLE bla(fid posint references foo); The intent here is that foo may contain negative numbers but that those rows won't be referenced by bla. This is similar to 1 and 2. Sure, I agree with you there are example where you may mean that. My point is about a ***warning***, because I think that most of the time this hides a future bug, even if some time it may be the intent. Have a nice day, -- Fabien Coelho - [EMAIL PROTECTED] ---(end of broadcast)--- TIP 5: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faqs/FAQ.html
Re: [HACKERS] [PATCHES] Foreign key type checking patch
On Tue, 2 Mar 2004, Fabien COELHO wrote:
Hello Stephan,
CREATE TABLE foo(fid INT4 NOT NULL PRIMARY KEY, ...);
CREATE TABLE bla(fid INT2 REFERENCES foo, ...);
The application will be fine till you enter fid=32767, and
it inserts will fail in bla with fid=32768. Much later on.
Which is fine if bla is meant to store a subset of the allowable foo
values.
[...]
Sure. This is NOT my point. I totally agree with you that the above
example MAY BE what the user intends, and that it must be allowed.
However it may ALSO be a bug that will pop up later on.
Although it is POSSIBLE that this is fine, it is much more PROBABLE that
it is a bug, hence I just suggest to issue a mere simple basic plain
user-friendly little warning, what is quite different from issuing an
error.
Thus, the user has the information. He may chose to go on as that is what
was meant, or maybe check the stuff and correct it.
In postgres compilation, gcc uses the -Wall option to issue warnings about
correct C constructs that may hide application bugs. This is the
philosophy I'm suggesting here for this very small feature.
Dear user, what you ask is right, however it looks a little bit strange,
so I tell you just in case. I'm sure you're pretty happy that the gcc
developers put such features for basic programmers, and that you use
them;-) Why not allowing that kind of approach in postgres?
Because producing noise warnings often *lower* the amount of use you get
from real warnings. If one has to wade through useless messages to divine
the ones that are meaningful, overall many people just start ignoring all
warnings. I could be convinced that it is notice material, since people
who don't want to see it probably don't want to see the other notices
either, but warning seems way to strong to me.
Fundamentally, I don't see a huge difference between this and
select * from foo,bla where foo.fid=bla.fid;
where the same general constraints on meaningful values apply.
CREATE TABLE foo(fid VARCHAR(4) NOT NULL PRIMARY KEY, ...);
CREATE TABLE bla(fid VARCHAR(8) REFERENCES foo, ...);
declaring a larger size is not a problem here, however you will
never be able to but any reference in bla larger than 4 as it must
match its counter part in foo. So it is just a little bit stupid.
This one is fairly pointless
Isn't it what I'm saying?
for the single column case but a multiple column match unspecified
constraint could allow the full 8 characters if there's a second column
which is null.
I do not understand. I can't see how you can put 8 characters in a
reference which must match a 4 characters string.
Because in match unspecified, any column being null means the remainder of
the columns are not checked against the other table. IE given a key of
(fid, other), ('abcdefg', null) is valid in match unspecified even if the
other key can only have 4 characters.
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Re: [HACKERS] [PATCHES] Foreign key type checking patch
Dear Stephan, Although it is POSSIBLE that this is fine, it is much more PROBABLE that it is a bug, hence I just suggest to issue a mere simple basic plain user-friendly little warning, what is quite different from issuing an error. [...] Why not allowing that kind of approach in postgres? Because producing noise warnings often *lower* the amount of use you get from real warnings. Ok, I understand this argument. I agree with the general idea... However I think that this case is special enough in the sense that: (1) it should occur rarely, and when it occurs (2) it should be most of the time appropriate. If one has to wade through useless messages to divine the ones that are meaningful, overall many people just start ignoring all warnings. I could be convinced that it is notice material, since people who don't want to see it probably don't want to see the other notices either, but warning seems way to strong to me. Well, I used NOTICE in my suggested implementation, so it is fine;-) Fundamentally, I don't see a huge difference between this and select * from foo,bla where foo.fid=bla.fid; where the same general constraints on meaningful values apply. 1/ The warning I'm suggesting is ONCE in the life-time of the table, when it is declared. Your above case would be EVERY TIME a join is issued on the tables, which is likely to be quite often! 2/ If you use a select with a join, and if your modelisation is correct, then you must have declared a foreign key on your table, so you already get the warning at that time. If you chose to ignore it, you must be right! 3/ If you bother to declare foreign keys, you want them to be sound, so if postgres can help a little bit, that should not harm. -- Fabien Coelho - [EMAIL PROTECTED] ---(end of broadcast)--- TIP 6: Have you searched our list archives? http://archives.postgresql.org
