Snippet Name: Disable_index_in_SQL

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Created Date: 2025-06-16 13:12:48
Last Modified Date: 2025-05-31 10:32:00

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Title: Disable index in SQL
Author: user
In SQL server, can I create a new index that is marked as disabled, or do i have to create it and disable it?

Author: ChatGPT
In SQL Server, you cannot directly create an index in a disabled state. Instead, you must first create the index and then disable it.

Here???s how you can do it:

1. **Create the index:**
   ```sql
   CREATE INDEX IX_YourIndexName ON YourTableName (YourColumnName);
   ```

2. **Disable the index:**
   ```sql
   ALTER INDEX IX_YourIndexName ON YourTableName DISABLE;
   ```

When an index is disabled, it remains defined in the database but is not usable for queries or maintenance operations. To make it usable again, you can rebuild it using:

```sql
ALTER INDEX IX_YourIndexName ON YourTableName REBUILD;
```

### Why this limitation?
SQL Server does not support directly creating a disabled index because the index needs to be built and populated with data before it can exist, even in a disabled state. Once populated, you can disable it to remove its impact on query plans or to prepare for certain maintenance tasks.

Author: user
i have a non clustered primary key, and when i look at the index stats, it does not seem to be used at all; the table certainly receives updates. is that normal? 

Author: ChatGPT
Yes, it's possible for a **non-clustered primary key** index to appear unused in your index statistics, even if the table is receiving updates. Here are some reasons why this could happen:

---

### 1. **Index Usage Statistics and Updates**
   - **`sys.dm_db_index_usage_stats`** only tracks **seeks, scans, and lookups** for queries. It does not record updates (inserts, updates, deletes) that affect the index.
   - Even if updates are happening on the table, they may not show up in the `user_updates` column of `sys.dm_db_index_usage_stats` for the primary key index if no query directly uses it for data retrieval.

---

### 2. **Primary Key Design and Query Patterns**
   - A **non-clustered primary key** may not be used if queries do not include it in WHERE clauses or JOIN conditions.
   - If other indexes cover the queries better or the primary key column is rarely queried directly, the optimizer may not choose to use the primary key index.

---

### 3. **Implications of Non-Clustered Primary Key**
   - If a **non-clustered primary key** is defined, updates to the table must still maintain the index, ensuring uniqueness. This maintenance is independent of query usage.
   - For tables with frequent updates, maintaining an unused index can add overhead without benefiting query performance.

---

### 4. **Clustered Index Considerations**
   - In many scenarios, a **clustered primary key** is preferred because it defines the physical order of the table and is more likely to be used by the query optimizer.
   - If the table has no clustered index, updates might still indirectly update the non-clustered primary key index, but it won't show significant activity in usage stats.

---

### Steps to Investigate Further

1. **Check Query Patterns**:
   Look at the queries hitting the table. Are they filtering or joining on the primary key column? If not, this could explain why the index is unused.

2. **Analyze the Index**:
   Use **Database Engine Tuning Advisor** or **Query Store** to evaluate whether the primary key index is truly unused or inefficiently designed for your workload.

3. **Consider Maintenance Costs**:
   If the index remains unused and query patterns do not justify its existence, consider dropping or redefining the primary key index. Ensure another mechanism enforces uniqueness if necessary.

---

### Example Query to Inspect Index Usage

```sql
SELECT 
    OBJECT_NAME(s.[object_id]) AS TableName,
    i.name AS IndexName,
    s.user_seeks, 
    s.user_scans, 
    s.user_lookups, 
    s.user_updates
FROM 
    sys.dm_db_index_usage_stats s
JOIN 
    sys.indexes i
    ON s.[object_id] = i.[object_id] 
    AND s.index_id = i.index_id
WHERE 
    OBJECT_NAME(s.[object_id]) = 'YourTableName'
    AND i.name = 'YourPrimaryKeyIndexName';
```

This query will help confirm whether the index is being used for seeks, scans, or lookups, or if its updates are simply from table modifications.

---

### Key Takeaway
If the primary key index is unused for query purposes but still incurs maintenance overhead, evaluate its necessity. Sometimes, redesigning the indexing strategy or making it a clustered index might better align with the table's usage patterns.