Re: Checking number of entries

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...

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