Author: Kevin Feasel

Eran Golan splits out a table:

Not long ago, I worked with a customer who was experiencing persistent blocking and occasional deadlocks in one of their core systems. The application itself wasn’t new, but over the years it had grown significantly. New features had been added, more processes were interacting with the database, and naturally the schema had evolved along the way.

One table in particular stood out. It had gradually grown to contain well over a hundred columns. Originally it had been designed to represent a single business entity in one place, which made the model easy to understand and query. But as more attributes were added over time, the table became increasingly wide.

Frankly, based off of Eran’s description, this sounds like a failure in normalizing the table appropriately. Normalization is not just about “There are many of X to one Y, so make two separate tables for X and Y.” In particular, 5th normal form (keys imply join dependencies) tells us that, if we can break out a table X into X1 and X2, and then join X1 and X2 together without losing any information or generating spurious new information, then 5NF requires we break it out. Eran is describing in narrative exactly that concept, though the description of how the customer broke that data out may or may not have satisfied 5NF.