Re: database design

From: --CELKO-- <>
Date: 7 May 2002 14:06:37 -0700
Message-ID: <>

Real programmers don't use auto-increment, IDENTITY, etc. in their real tables.

The IDENTITY column is a hold over from the early programming language which were <i>very<i> close to the hardware. For example, the fields in a COBOL or FORTRAN program were assumed to be physically located in main storage in the order they were declared in the program. This meant that you could define a template that overlaid the same physical space and read the representation in several different ways. In COBOL, the command was REDEFINES, EQUIVALENCE in FORTRAN and a union in 'C'.

From a logical viewpoint, this redefinition makes no sense at all. It is confusing the numeral with the number the numeral represents. The history of programming after this point in time has been to divorce the logical and physical models completely.

The early SQLs were based on existing file systems. The data was kept in physically contiguous disk pages, in physically contiguous rows, made up of physically contiguous columns. In short, just like a deck of punch cards or a magnetic tape.

But physically contiguous storage is only one way of building a relational database and it is not always the best one. But aside from that, the whole idea of a relational database is that user is not supposed to know how things are stored at all, much less write code that depends on the particular physical representation in a particular release of a particular product.

One of the biggest errors is the IDENTITY column in the Sybase family (SQL Server and Sybase). People actually program with this "feature" and even use it as the primary key for the table! Now, let's go into painful details as to why this thing is bad.

The practical considerations are that IDENTITY is proprietary and non-portable, so you know that you will have maintenance problems when you change releases or port your system to other products. It also has some very strange bugs in both Sybase and SQL Server; go to a newsgroup and do a search.

But let's look at the logical problems. First try to create a table with two columns and try to make them both IDENTITY columns. If you cannot declare more than one column to be of a certain datatype, then that thing is not a datatype at all, by definition.

Next, create a table with one column and make it an IDENTITY column. Now try to insert, update and delete different numbers from it. If you cannot insert, update and delete rows from a table, then it is not a table by definition.

Finally create a simple table with one IDENTITY column and a few other columns. Use a few statements like

INSERT INTO Foobar (a, b, c) VALUES  ('a1', 'b1', 'c1'); 
INSERT INTO Foobar (a, b, c) VALUES  ('a2', 'b2', 'c2'); 
INSERT INTO Foobar (a, b, c) VALUES  ('a3', 'b3', 'c3'); 

to put a few rows into the table and notice that the IDENTITY column sequentially numbered them in the order they were presented. If you delete a row, the gap in the sequence is not filled in and the sequence continues from the highest number that has ever been used in that column in that particular table.

But now use a statement with a query expression in it, like this:

INSERT INTO Foobar (a, b, c)
SELECT x, y, z
  FROM Floob;

Since a query result is a table, and a table is a set which has no ordering, what should the IDENTITY numbers be? The entire, whole, completed set is presented to Foobar all at once, not a row at a time.  There are (n!) ways to number (n) rows, so which one do you pick? The answer has been to use whatever the physical order of the result set happened to be. That non-relational phrase "physical order" again.

But it is actually worse than that. If the same query is executed again, but with new statistics or after an index has been dropped or added, the new execution plan could bring the result set back in a different physical order. Can you explain from a logical model why the same rows in the second query get different IDENTITY numbers? In the relational model, they should be treated the same if all the values of all the attributes are identical.

Think about trying to do replication on two databases that differ only by an index, or by cache size or something that occasionally gives them different execution plans for the same statements. Want to try to maintain such a system?

There are better ways of creating identifiers.

If you'd like a unique number generator that will fill in the gaps, use UDFs in SQL Server 2000 to create new IDENTITY functionality. The details are in an article by Fernando G. Guerrero Received on Tue May 07 2002 - 23:06:37 CEST

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