There are times when you as a DBA wished you were involved before someone designed a data model. After release to production correcting mistakes is usually complicated and with risks.

In a earlier post about legacy databases I mentioned that dirty data are more likely to enter the database on an earlier stage. One day while I was looking at the selectivity for a few columns I found the values a bit strange. After some digging I noticed that for most of the time the data was OK, even though a proper foreign key constraint was missing for the columns. But at some time many years ago somebody had inserted something completely different, which had clearly affected the selectivity…