title: What is cardinality and how does it affect performance (SQL Server)?
body: <p>We have a massive table where I need to make an update on a single row. I don't know the primary key of the row, but I have a varchar value that is unique within that table. I also have the value of a few other columns in that tables.</p>
  
  <p>Running the update is taking upwards of three minutes, and I assume its doing a full table scan.</p>
  
  <p>Looking at the indices on the table, the index on the column has a cardinality of zero and page count of zero. There are other indices with a cardinality equal to the number of rows in the table (couple million) and page count of a couple hundred thousand.</p>
  
  <p>What do these numbers actually mean? </p>
  
  <p>And as a followup, would adding a restriction that hits an index with a higher cardinality or number of pages speed up the execution? Or is there anything else I can look at on the indicies to find one that would be better suited to quickly find the row I ned to change.</p>