use a list instead of a set for index changes to perserve order

dbt/adapters/postgres/relation.py

@@ -1,5 +1,5 @@
 from dataclasses import dataclass, field
-from typing import FrozenSet, Optional, Set
+from typing import FrozenSet, List, Optional
 
 from dbt.adapters.base.relation import BaseRelation
 from dbt.adapters.contracts.relation import RelationConfig, RelationType
@@ -79,7 +79,7 @@ def _get_index_config_changes(
         self,
         existing_indexes: FrozenSet[PostgresIndexConfig],
         new_indexes: FrozenSet[PostgresIndexConfig],
-    ) -> Set[PostgresIndexConfigChange]:
+    ) -> List[PostgresIndexConfigChange]:
         """
         Get the index updates that will occur as a result of a new run
 
@@ -90,18 +90,22 @@ def _get_index_config_changes(
         3. Index is old -> drop these
         4. Indexes are not equal -> drop old, create new -> two actions
 
-        Returns: a set of index updates in the form {"action": "drop/create", "context": <IndexConfig>}
+        *Note:*
+            The order of the operations matters here because if the same index is dropped and recreated
+            (e.g. via --full-refresh) then we need to drop it first, then create it.
+
+        Returns: an ordered list of index updates in the form {"action": "drop/create", "context": <IndexConfig>}
         """
-        drop_changes = set(
+        drop_changes = [
             PostgresIndexConfigChange.from_dict(
                 {"action": RelationConfigChangeAction.drop, "context": index}
             )
             for index in existing_indexes.difference(new_indexes)
-        )
-        create_changes = set(
+        ]
+        create_changes = [
             PostgresIndexConfigChange.from_dict(
                 {"action": RelationConfigChangeAction.create, "context": index}
             )
             for index in new_indexes.difference(existing_indexes)
-        )
-        return set().union(drop_changes, create_changes)  # type: ignore
+        ]
+        return drop_changes + create_changes

dbt/adapters/postgres/relation_configs/materialized_view.py

@@ -101,12 +101,12 @@ def parse_relation_results(cls, relation_results: RelationResults) -> dict:
 
 @dataclass
 class PostgresMaterializedViewConfigChangeCollection:
-    indexes: Set[PostgresIndexConfigChange] = field(default_factory=set)
+    indexes: List[PostgresIndexConfigChange] = field(default_factory=list)
 
     @property
     def requires_full_refresh(self) -> bool:
         return any(index.requires_full_refresh for index in self.indexes)
 
     @property
     def has_changes(self) -> bool:
-        return self.indexes != set()
+        return self.indexes != []