ALTER TABLE DW

• Hi smpa01 

I understand the frustration. This is a known limitation in Fabric Data Warehouse as of today.

Root Cause:

Fabric DW is built on Synapse Dedicated SQL Pool architecture. In this engine, `ALTER TABLE ALTER COLUMN` is not supported. That’s why you are getting:

`Msg 2401, Level 16, State 2, Line 21 - The specified ALTER TABLE statement is not supported`

What IS supported vs NOT supported:

| Operation | Status in Fabric DW |

| --- | --- |

| `ALTER TABLE ADD COLUMN` | Supported |

| `ALTER TABLE DROP COLUMN` | Supported |

| `ALTER TABLE ALTER COLUMN` | Not Supported |

| `ALTER TABLE RENAME` | Not Supported |

**Recommended Workaround - CTAS Pattern:**

Microsoft recommends using `CREATE TABLE AS SELECT` to change data types. Here is the full flow for your case:

```sql

Step 1: Create new table with updated schema

CREATE TABLE [dbo].[test_log_new]

WITH

(

    DISTRIBUTION = ROUND_ROBIN,

    CLUSTERED COLUMNSTORE INDEX

)

AS

SELECT

    CAST([row_sum] AS DECIMAL(38,6)) AS [row_sum], -- changed data type

    [col2],

    [col3],

    CAST(NULL AS VARCHAR(50)) AS [ingestion_engine] -- new column added

FROM [dbo].[test_log];

Step 2: Validate data in new table

SELECT TOP 100 * FROM [dbo].[test_log_new];

Step 3: Swap tables

DROP TABLE [dbo].[test_log];

EXEC sp_rename 'dbo.test_log_new', 'test_log';