Thursday, April 14, 2005

On referential integrity

At my place of employment we have a table that looks like this:

CREATE TABLE permissions (
    id character(40) NOT NULL,
    userid character(40) REFERENCES USERS(id),
    objectid character(40) NOT NULL,
    permission character varying(15) NOT NULL
);
Note that objectid isn't a foreign key to anything. (Before I joined there was no foreign key declared for userid, either.) That is because in this schema, everything gets a unique char(40) ID, and for flexibility the designer wanted to use the same permissions storage for all tables in the database.

(The fix for this, BTW, would involve creating an "objects" table that simply held all the IDs in the system and have that referenced by both the objects tables and any table like this one that wants to be able to reference "any object." I haven't done that yet, but I'll be moving that up on my priority list now.)

About a month ago, after suitable testing, I ran an upgrade script against our live database that went something like this:

CREATE TABLE alerttypes_old AS SELECT * FROM alerttypes;
DROP TABLE alerrtypes;
-- create new table
-- insert into new table massaged data from _old

As it happens, alerttypes is one of the tables that we are interested in permissions for. I forgot to delete the appropriate entries corresponding to the old rows, but worse, I forgot to create new ones for the new alerttypes.

What makes this more surprising is that the developer who reviewed the script missed this too. But that is what happens when you don't have proper integrity constraints: sooner or later, you're going to be restoring from backup. Even if you're a smart guy. Even if you test first (on several machines). Even if you have code reviews.

Incidently, I have seen people leave out FK constraints (or drop the ones I added -- grr!) to accomodate series of statements that temporarily violate the constraint, but eventually (in theory) leave things in a correct state. The correct course here is to put your related statements in a transaction (a good idea anyway), and tell your database to check constraints when the transaction ends, not before. For postgresql, that looks like this:

acs=# begin work;
BEGIN
acs=# SET CONSTRAINTS ALL DEFERRED;
SET CONSTRAINTS
-- ...

No comments: