microsoft_sql_server_cdc: use cluster index when primary key is missing

Hi,

When using snapshot we can use cluster index instead a primary key.
We can try additional query here to search cluster index.

As alternative, to add config field snapshot_order_columns that defines order by columns.

To find cluster index:

SELECT t.name,  c.name FROM sys.indexes AS i
  JOIN sys.index_columns ic ON i.object_id = ic.object_id AND i.index_id = ic.index_id
  JOIN sys.columns c ON ic.object_id = c.object_id AND ic.column_id = c.column_id
  JOIN sys.tables t ON i.object_id = t.object_id
  JOIN sys.schemas s ON t.schema_id = s.schema_id
WHERE i.type = 1 -- 1 is cluster
 AND s.name = ? -- schema
 AND t.name = ? -- table
ORDER BY ic.key_ordinal;

Regards

Start in internal/impl/mssqlserver/replication/snapshot.go at the linked lookup around line 251, and trace how snapshot ordering is chosen when a primary key is absent. Use the proposed sys.indexes query as the reference for finding clustered-index columns. Done means snapshots can use the clustered index when no primary key exists.