A map source references two schemas: a connection schema and a map schema. When you connect for the first time, the connection schema is stored as the map schema. After that, when you reload a map and reconnect, the schema stored in the map is compared with the connection schema to check for any mismatches (indicating that the schema has changed since the map was last saved). The same process occurs when you change tables. If the connection schema differs from the stored map schema, an error is displayed related to missing fields. The new fields are not highlighted because they haven't been mapped yet, but the user is alerted about the missing fields, as any target expressions that are mapped to these fields will be lost.

To resolve schema mismatches (after changing the table, attempting to reconnect, and receiving the error), follow one of these steps:

This also applies to single-mode targets. However, for targets with the replace output mode, schema mismatch is not a concern, as the map schema is applied.

For multi-mode targets, schemas are resolved by pushing the map schema to the connector. While the schemas may not strictly match (and likely won't), it's assumed that the fields in the schema will align with the corresponding fields in the actual target.