FIX: AV on INSERT of UNIONed SELECTs to Table With DEFAULTs

• Microsoft SQL Server, versions 6.0 and 6.5

SYMPTOMS

INSERTing to a table from a UNION of two or more SELECT statements can result in an Access Violation.

CAUSE

At a certain threshold of data, a work table needs to be created to resolve the UNION correctly. If the target table has DEFAULTs, these are not applied to intermediate work tables but other attributes of the target table, such as NOT NULL, are applied, and this would cause the query to fail with a different error if there was not the AV.

WORKAROUND

Implement the INSERT as distinct INSERT SELECTs for each table in the UNION. If there is a UNIQUE INDEX on the target table, use the IGNORE_DUP_KEY option on that INDEX to filter out duplicate rows that would previously have been filtered out by the UNION. In many cases, this method can be much faster anyway because it can eliminate one or more intermedate work tables.

STATUS

Microsoft has confirmed this to be a problem in Microsoft SQL Server version 6.5. This problem has been corrected in U.S. Service Pack 1 for Microsoft SQL Server version 6.5. For more information, contact your primary support provider.