Troubleshooting Guide: SQL Server Error 4950 – ALTER TABLE SWITCH Partition Failure

Why This Error Happens

Error 4950 occurs when the partition number referenced in the ALTER TABLE ... SWITCH statement does not exist on the target or source table’s partition scheme. This typically happens because the partition function boundaries were altered (merged or split) after the table was created, causing a mismatch between the expected partition count and the actual partition scheme mapping. It can also occur if the table was created with a different partition scheme than the one currently applied, or if a stale execution plan/cached metadata references an outdated partition layout.

Step-by-Step Resolution Guide

Step 1: Verify the Current Partition Scheme and Function

• Run the following query to inspect existing partitions on the table in question:

  SELECT p.partition_number, p.rows, ps.name AS PartitionScheme
  FROM sys.partitions p
  JOIN sys.indexes i ON p.object_id = i.object_id AND p.index_id = i.index_id
  JOIN sys.data_spaces ds ON i.data_space_id = ds.data_space_id
  JOIN sys.partition_schemes ps ON ds.data_space_id = ps.data_space_id
  WHERE p.object_id = OBJECT_ID('YourTableName');

• Confirm the partition number specified in your SWITCH statement actually exists in the output.

Step 2: Validate Partition Function Boundary Values

• Check the partition function definition using:

  SELECT * FROM sys.partition_range_values 
  WHERE function_id = (SELECT function_id FROM sys.partition_functions WHERE name = 'YourPartitionFunction');

• Compare these boundary values against the boundaries you expect in your SWITCH logic — a recent MERGE RANGE or SPLIT RANGE operation may have shifted partition numbers.

Step 3: Recreate or Adjust the Partition Scheme if Needed

• If the partition scheme is out of sync with the function, drop and recreate the scheme:

  DROP PARTITION SCHEME YourPartitionScheme;
  CREATE PARTITION SCHEME YourPartitionScheme 
  AS PARTITION YourPartitionFunction 
  ALL TO ([PRIMARY]);

• Ensure both the source and target tables in the SWITCH operation are aligned to the same partition function and filegroup structure.

Step 4: Rebuild the Table with Correct Partition Alignment

• If misalignment persists, rebuild the affected table using CREATE TABLE ... ON YourPartitionScheme(ColumnName) to force correct partition mapping.
• Use sp_help 'YourTableName' afterward to confirm the new partition number sequence matches your switch statement.

Step 5: Update Statistics and Clear Cached Plans

• Run UPDATE STATISTICS YourTableName; to refresh partition-level metadata.
• Clear any stale cached plans with DBCC FREEPROCCACHE; (use cautiously in production) to ensure SQL Server re-evaluates the current partition layout.

Alternative or Verification Steps

• Re-run the original ALTER TABLE ... SWITCH PARTITION statement after confirming partition numbers match between source and target tables — it should now execute without Error 4950.
• As an alternative, use SQL Server Management Studio (SSMS) to visually inspect the table’s Storage > Partitions tab under Table Properties, which provides a GUI view of partition numbers and boundary values for quick cross-verification.