Re: Checking number of entries - Mailing list pgsql-general

At 01:53 29/09/00 -0400, Tom Lane wrote:
>
>The canonical example is a check like
>    CHECK (mycol > (SELECT max(othertab.othercol) FROM othertab))
>declared as a constraint on mytab.

I would argue that a CHECK constraint only applies to the table on which it
is defined - basically a contract to ensure that certain conditions apply
to changes on that table.

Once you allow functions in constraints, you give up all hope of
cross-compatibility, eg.

Create Table tab1 (f1 integer);
Create Function tot_of_tab1() returns integer
     as 'select cast(sum(f1) as int4) from tab1' language 'sql';
Alter Table tab1 add check(tot_of_tab1() > 0);

zzz=# insert into tab1 values(1);
INSERT 142380 1
zzz=# insert into tab1 values(-10);
INSERT 142381 1
zzz=# select tot_of_tab1();
 tot_of_tab1
-------------
          -9
(1 row)

zzz=# insert into tab1 values(-12);
ERROR:  ExecAppend: rejected due to CHECK constraint $1

This demonstrates the problem, and, I think, highlights a bug in the
constraint checking code: the constraint should have failed on the second
insert. Maybe the constraint is evaluate before the insert?

>Should this mean that the constraint on mytab can prevent insertions
>into othertab?

I'd say not. Consider:

   weird_function() returns boolean as 'select current_time > ''16:00:00''';

This *could* be used to prevent updates occurring before 4pm - it says
nothing about the validity of the data, it is just a rule about how updates
can be performed.

Does DB theory say what sort of contract CHECK constraints imply?

>You can devise related paradoxes within a single table if constraints
>involving aggregates are permitted.  The basic problem is that it's
>not clear which data values are the constrainers and which are the
>constrainees.

What's wrong with CHECK constraints only affecting the table on which they
are defined? It seems better than (a) allowing back-door functions, and (b)
preventing more sensible subselects.

Sorry if this ground has already been covered...


----------------------------------------------------------------
Philip Warner                    |     __---_____
Albatross Consulting Pty. Ltd.   |----/       -  \
(A.B.N. 75 008 659 498)          |          /(@)   ______---_
Tel: (+61) 0500 83 82 81         |                 _________  \
Fax: (+61) 0500 83 82 82         |                 ___________ |
Http://www.rhyme.com.au          |                /           \|
                                 |    --________--
PGP key available upon request,  |  /
and from pgp5.ai.mit.edu:11371   |/