Property skew

When most but not all rows in the table have the same value for a property, the column values are skewed. Property skew might cause a query not to use a database index.

Because of skew for Property1, for example, a query execution plan might not use the database index on that property in the following query:

SELECT Id 
FROM Document 
WHERE Property1 = 'value'

Whether the index is used depends on the following circumstances:

The statistics and configuration that is available in the database
The value that is searched on when the query execution plan is cached