Chris Date releases free book chapterJune 18, 2005
Chris Date, co-founder of the relational database model has just
released a free chapter from his latest book "Database In Depth" by
Back in the 1970's when Codd and Date first developed the rules of
normalization, disk was very expensive and the goal of relational
demoralization was the elimination of redundancy. Today, disk
is a cheap commodity, and many of the original rules of
normalization no longer apply because redundancy is cheaper than the
run-time performance overhead of joining tables together
Also, vendors such as Oracle have extended their relational
offerings by offering data structures that violates Dates original
rules of normalization:
1 - The ability to create
non-first-normal-form tables (i.e. tables with repeating groups
via PL/SQL varray data types).
2 - The ability to hard-link
rows via pointers, using constructs such as Oracle nested tables
and stored arrays of pointers (Object ID's).
3 - The ability to incorporate
physical storage rules into the structures, such as the APPEND
hint, and the Oracle cluster table structure, where related
table rows can be placed on adjacent data blocks, and index
organized tables, where the table data is stored inside the
index tree structure.
In the sample chapter, Date notes
that his relational model condemns these practices:
The fact that the
relational model says nothing about physical storage is
deliberate, of course. The idea was to give implementers the
freedom to implement the model in whatever way they chose—in
particular, in whatever way seemed likely to yield good
performance—without compromising on data independence.
The sad fact is, however,
that SQL vendors seem mostly not to have understood this point;
instead, they map base tables fairly directly to physical
storage,* and (as noted in the previous section) their products
therefore provide far less data independence than relational
systems are theoretically capable of.