[Bug] Use a list instead of a set for index changes to perserve order

.changes/unreleased/Fixes-20240425-133401.yaml

@@ -0,0 +1,6 @@
+kind: Fixes
+body:  Replace usage of `Set` with `List` to fix issue with index updates intermittently happening out of order
+time: 2024-04-25T13:34:01.018399-04:00
+custom:
+  Author: mikealfare
+  Issue: "72"

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 != []

tests/unit/test_materialized_view.py

@@ -0,0 +1,60 @@
+from copy import deepcopy
+
+from dbt.adapters.contracts.relation import RelationType
+from dbt.adapters.relation_configs.config_change import RelationConfigChangeAction
+
+from dbt.adapters.postgres.relation import PostgresRelation
+from dbt.adapters.postgres.relation_configs import PostgresIndexConfig
+
+
+def test_index_config_changes():
+    index_0_old = {
+        "name": "my_index_0",
+        "column_names": {"column_0"},
+        "unique": True,
+        "method": "btree",
+    }
+    index_1_old = {
+        "name": "my_index_1",
+        "column_names": {"column_1"},
+        "unique": True,
+        "method": "btree",
+    }
+    index_2_old = {
+        "name": "my_index_2",
+        "column_names": {"column_2"},
+        "unique": True,
+        "method": "btree",
+    }
+    existing_indexes = frozenset(
+        PostgresIndexConfig.from_dict(index) for index in [index_0_old, index_1_old, index_2_old]
+    )
+
+    index_0_new = deepcopy(index_0_old)
+    index_2_new = deepcopy(index_2_old)
+    index_2_new.update(method="hash")
+    index_3_new = {
+        "name": "my_index_3",
+        "column_names": {"column_3"},
+        "unique": True,
+        "method": "hash",
+    }
+    new_indexes = frozenset(
+        PostgresIndexConfig.from_dict(index) for index in [index_0_new, index_2_new, index_3_new]
+    )
+
+    relation = PostgresRelation.create(
+        database="my_database",
+        schema="my_schema",
+        identifier="my_materialized_view",
+        type=RelationType.MaterializedView,
+    )
+
+    index_changes = relation._get_index_config_changes(existing_indexes, new_indexes)
+
+    assert isinstance(index_changes, list)
+    assert len(index_changes) == len(["drop 1", "drop 2", "create 2", "create 3"])
+    assert index_changes[0].action == RelationConfigChangeAction.drop
+    assert index_changes[1].action == RelationConfigChangeAction.drop
+    assert index_changes[2].action == RelationConfigChangeAction.create
+    assert index_changes[3].action == RelationConfigChangeAction.create