This is a specific limitation of the SQL Server computed column matching logic, when an outermost ISNULL is used, and the datatype of the column is bit.
To avoid the issue, any of the following workarounds may be employed:
- Do not use an outermost
ISNULL(the only way to make a computed columnNOT NULL). - Do not use the
bitdata type as the final type of the computed column. - Make the computed column
PERSISTEDand enable trace flag 176.
Details
The heart of the issue is that without trace flag 176, all computed column references in a query (even persisted) are always expanded into the underlying definition very early in query compilation.
The idea of expansion is that it could enable simplifications and rewrites that can only work on the definition, not on the column name alone. For example, there may be predicates in the query referencing that computed column that could make part of the calculation redundant, or otherwise more constrained.
Once early simplifications and rewrites are considered, query compilation attempts to match expressions in the query to computed columns (all computed columns, not only those originally found in the query text).
Unchanged computed column expressions match back to the original computed column without issue in most cases. There appears to be a bug when specific to matching an expression of bit type, with an outermost ISNULL. Matching is unsuccessful in this specific case, even where a detailed examination of the internals shows that it should succeed.
