Occasionally, a database can become corrupt due to hardware failures or system failures. In such cases, the best solution is to restore the database from a known good backup. However, if a backup is not available, repairs will have to be made to the database to make it functional again. This process may cause data loss, and it can be difficult to identify the extent of the loss. In such situations, accessing data from non-clustered indexes can be a helpful approach.
Non-clustered indexes in SQL Server store data separately from the base table, allowing for multiple sort orders and improved performance. Unlike clustered indexes, which store data along with a pointer to the row in the table, non-clustered indexes provide added flexibility and performance benefits.
Let’s consider a scenario where corruption has affected the data between RowID 10000 and 20000 in the base table. As non-clustered indexes store a duplicate copy of the data, it is still accessible through the indexes. However, attempting to retrieve the data directly from the base table or insert new data within the affected range will fail due to the corruption.
To retrieve the data from the non-clustered indexes, we can utilize the “With (Index=_)” query hint to force the optimizer to use the index instead of the table. This can aid in rebuilding the base table and recovering the displaced data.
For example, if we have non-clustered indexes with index IDs 1, 2, 3, and 4, we can write queries to pull data from the index data pages instead of the table. By using the appropriate index ID, we can retrieve the desired data without performing a row lookup from the base table.
Here is an example of a two-query method using the non-clustered index NC_IndexB and the composite index NC_IndexD:
SELECT RowID, KeywordB FROM Example WITH (INDEX = 2) WHERE RowID = 10234; /* Returns RowID and KeywordB values where RowID = 10234 from NC_IndexB */ SELECT RowID, KeywordA, KeywordC FROM Example WITH (INDEX = 4) WHERE RowID = 10234; /* Returns RowID, KeywordA, and KeywordC where RowID = 10234 from NC_IndexD */
Since these queries are satisfied by the data in the index, there is no need for a row lookup from the base table. The retrieved data can be inserted into a temporary table or the base table of the repaired database, allowing access through normal methods in the future.
While this process may not be easy or quick for large systems, it can be a lifesaver in situations where data loss is imminent. Having tools available to recover data when all else fails can make a significant difference in minimizing the impact of a database corruption.
It is important to note that extracting the index ID information for a given table can be done using the following script:
SELECT sysobjects.id AS 'TableObjectID', sysobjects.Name AS 'TableName', sysindexes.indid AS 'IndexID', sysindexes.name AS 'IndexName', syscolumns.name AS 'ColumnName', syscolumns.colorder AS 'ColumnOrder' FROM sysindexes INNER JOIN sysobjects ON sysindexes.id = sysobjects.id INNER JOIN syscolumns ON syscolumns.id = sysindexes.id WHERE sysobjects.name = 'Example' -- Table where data is needed AND sysindexes.indid <> 0 -- Eliminate base table
By understanding the index design and utilizing non-clustered indexes effectively, it is possible to recover data from a corrupt database and minimize the impact of data loss.
Remember, prevention is always better than cure. Regularly backing up your databases and implementing proper monitoring and maintenance practices can help avoid such situations and ensure the integrity of your data.