why can't I elastic resize from dc2.large 4 nodes to ra3.large 1 node

There are a few reasons why you may not be able to perform an elastic resize from a DC2.large 4 node cluster to a RA3.large 1 node cluster in Amazon Redshift:

1. Hardware Incompatibility: The DC2 and RA3 node types are based on different hardware architectures. The DC2 nodes use older generation hardware, while the RA3 nodes use newer, more powerful hardware. Redshift does not allow direct resizing between incompatible node types.
2. Node Count Mismatch: You are trying to resize from a 4-node DC2.large cluster to a 1-node RA3.large cluster. Redshift requires that the node count remains the same during an elastic resize operation. You cannot change the node count as part of an elastic resize.
3. Capacity Constraints: The RA3.large node type has different storage and compute capacities compared to the DC2.large node type. The system may not be able to accommodate the capacity requirements of your 4-node DC2.large cluster on a single RA3.large node.

To perform the type of resize you're attempting, you would need to go through a full cluster resize operation, which involves the following steps:

1. Create a new Redshift cluster with the desired RA3.large node type and node count.
2. Migrate your data from the existing DC2.large cluster to the new RA3.large cluster using a tool like AWS Database Migration Service (DMS) or a manual backup/restore process.
3. Update your application to point to the new RA3.large cluster.
4. Decommission the old DC2.large cluster. This full cluster resize process allows you to change both the node type and node count, but it requires more planning and downtime compared to an elastic resize operation.