Concurrency Considerations for Fast Index Creation

While a secondary index is being created or dropped, the table is locked in shared mode. Any writes to the table are blocked, but the data in the table can be read. When you alter the clustered index of a table, the table is locked in exclusive mode, because the data must be copied. Thus, during the creation of a new clustered index, all operations on the table are blocked.

Before it can start executing, a CREATE INDEX or ALTER TABLE statement must always wait for currently executing transactions that are accessing the table to commit or rollback before it can proceed. In addition, ALTER TABLE statements that create a new clustered index must wait for all SELECT statements that access the table to complete (or their containing transactions to commit). Even though the original index exists throughout the creation of the new clustered index, no transactions whose execution spans the creation of the index can be accessing the table, because the original table must be dropped when clustered index is restructured.

Once a CREATE INDEX or ALTER TABLE statement that creates a secondary index begins executing, queries can access the table for read access, but cannot update the table. If an ALTER TABLE statement is changing the clustered index, all queries must wait until the operation completes.

A newly-created secondary index contains only the committed data in the table at the time the CREATE INDEX or ALTER TABLE statement begins to execute. It does not contain any uncommitted values, old versions of values, or values marked for deletion but not yet removed from the old index.

Because a newly-created index contains only information about data current at the time the index was created, queries that need to see data that was deleted or changed before the index was created cannot use the index. The only queries that could be affected by this limitation are those executing in transactions that began before the creation of the index was begun. For such queries, unpredictable results could occur. Newer queries can use the index.

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