Online ALTER COLUMN final Sch-M lock duration bounds under concurrent DML in SQL Server 2019
Context SQL Server 2016 introduced online ALTER COLUMN for certain data type changes, allowing concurrent reads and writes during the operation. For incompatible conversions (e.g., int to bigint ), a data-copy phase occurs followed by a brief schema modification (Sch-M) lock to switch metadata. Unresolved behavior The documentation does not specify a maximum
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