Improve migration support using CREATE INDEX ASYNC (#47)

This PR improves migration support by using the `CREATE INDEX ASYNC`
syntax in `schema.py`.

Changes extracted and modified from #46.

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copy, and redistribute this contribution, under the terms of your
choice.

The changes are detailed below.

### `sql_create_index`

Previously defined as:

```python
sql_create_index = (
    "CREATE INDEX %(name)s ON %(table)s%(using)s "
    "(%(columns)s)%(include)s%(extra)s%(condition)s"
)
```

Now defined as:

```python
sql_create_index = (
    "CREATE INDEX ASYNC %(name)s ON %(table)s%(using)s "
    "(%(columns)s)%(include)s%(extra)s%(condition)s"
)
```

This matches the syntax defined by [Asynchronous indexes in Aurora
DSQL](https://docs.aws.amazon.com/aurora-dsql/latest/userguide/working-with-create-index-async.html).

### `sql_create_unique`

Previously defined as:

```python
sql_create_unique = (
    "ALTER TABLE %(table)s ADD CONSTRAINT %(name)s "
    "UNIQUE%(nulls_distinct)s (%(columns)s)%(deferrable)s"
)
```

Now defined as:

```python
sql_create_unique = "CREATE UNIQUE INDEX ASYNC %(name)s ON %(table)s (%(columns)s)"
```

This avoids the `ALTER TABLE ... ADD CONSTRAINT` syntax which is not
supported by DSQL according to [Supported subsets of SQL commands in
Aurora
DSQL](https://docs.aws.amazon.com/aurora-dsql/latest/userguide/working-with-postgresql-compatibility-supported-sql-subsets.html#alter-table-syntax-support).

### General cleanup

As part of this change I added some more documentation, and reorganized
the position of a few definitions to better isolate concepts. This will
make upcoming changes simpler too thanks to the improved consistency.

Author: danielfrankcom

aurora_dsql_django/schema.py

@@ -22,35 +22,38 @@
 
 
 class DatabaseSchemaEditor(schema.DatabaseSchemaEditor):
-    # The PostgreSQL backend uses "SET CONSTRAINTS ... IMMEDIATE" before
-    # "ALTER TABLE..." to run any any deferred checks to allow dropping the
-    # foreign key in the same transaction. This doesn't apply to Aurora DSQL.
-    sql_delete_fk = ""
+    """
+    Aurora DSQL schema editor based on the PostgreSQL backend.
+    
+    Aurora DSQL is PostgreSQL-compatible but supports a subset of PostgreSQL
+    operations. This class overrides SQL templates and methods to work within
+    DSQL's constraints.
+    """
 
-    # ALTER TABLE ADD CONSTRAINT PRIMARY KEY is not supported
-    sql_create_pk = ""
+    # Use DSQL's async index creation syntax.
+    sql_create_index = (
+        "CREATE INDEX ASYNC %(name)s ON %(table)s%(using)s "
+        "(%(columns)s)%(include)s%(extra)s%(condition)s"
+    )
+
+    # Create unique constraints as unique indexes instead of using "ALTER TABLE".
+    sql_create_unique = "CREATE UNIQUE INDEX ASYNC %(name)s ON %(table)s (%(columns)s)"
 
-    # "ALTER TABLE ... DROP CONSTRAINT ..." not supported for dropping UNIQUE
-    # constraints; must use this instead.
+    # Delete unique constraints by dropping the underlying index.
     sql_delete_unique = "DROP INDEX %(name)s CASCADE"
 
-    # The PostgreSQL backend uses "SET CONSTRAINTS ... IMMEDIATE" after this
-    # statement. This isn't supported by Aurora DSQL.
+    # Remove constraint management from default updates.
     sql_update_with_default = (
         "UPDATE %(table)s SET %(column)s = %(default)s WHERE %(column)s IS NULL"
     )
 
-    # ALTER TABLE ADD CONSTRAINT is not supported
-    sql_create_unique = ""
-
-    # ALTER TABLE ADD CONSTRAINT FOREIGN KEY is not supported
+    # These "ALTER TABLE" operations are not supported.
+    sql_create_pk = ""
     sql_create_fk = ""
-    # ALTER TABLE ADD CONSTRAINT CHECK is not supported
+    sql_delete_fk = ""
     sql_create_check = ""
     sql_delete_check = ""
-    # ALTER TABLE DROP CONSTRAINT is not supported
     sql_delete_constraint = ""
-    # ALTER TABLE DROP COLUMN is not supported
     sql_delete_column = ""
 
     def __enter__(self):

aurora_dsql_django/tests/unit/test_schema.py

@@ -13,14 +13,17 @@ def setUp(self):
     def test_sql_attributes(self):
         self.assertEqual(self.schema_editor.sql_delete_fk, "")
         self.assertEqual(self.schema_editor.sql_create_pk, "")
+        self.assertEqual(self.schema_editor.sql_create_index,
+                         "CREATE INDEX ASYNC %(name)s ON %(table)s%(using)s (%(columns)s)%(include)s%(extra)s%(condition)s")
+        self.assertEqual(self.schema_editor.sql_create_unique,
+                         "CREATE UNIQUE INDEX ASYNC %(name)s ON %(table)s (%(columns)s)")
         self.assertEqual(
             self.schema_editor.sql_delete_unique,
             "DROP INDEX %(name)s CASCADE")
         self.assertEqual(
             self.schema_editor.sql_update_with_default,
             "UPDATE %(table)s SET %(column)s = %(default)s WHERE %(column)s IS NULL"
         )
-        self.assertEqual(self.schema_editor.sql_create_unique, "")
         self.assertEqual(self.schema_editor.sql_create_fk, "")
         self.assertEqual(self.schema_editor.sql_create_check, "")
         self.assertEqual(self.schema_editor.sql_delete_check, "")