'Adding indexes for all foreign keys' docs

docs/cli.rst

@@ -310,6 +310,15 @@ If you omit the other table and other column references ``sqlite-utils`` will at
 
 See :ref:`python_api_add_foreign_key` in the Python API documentation for further details, including how the automatic table guessing mechanism works.
 
+.. _cli_index_foreign_keys:
+
+Adding indexes for all foreign keys
+-----------------------------------
+
+If you want to ensure that every foreign key column in your database has a corresponding index, you can do so like this::
+
+    $ sqlite-utils index-foreign-keys books.db
+
 .. _cli_defaults_not_null:
 
 Setting defaults and not null constraints

docs/python-api.rst

@@ -419,6 +419,17 @@ Here's an example adding two foreign keys at once:
         ("dogs", "home_town_id", "towns", "id")
     ])
 
+.. _python_api_index_foreign_keys:
+
+Adding indexes for all foreign keys
+-----------------------------------
+
+If you want to ensure that every foreign key column in your database has a corresponding index, you can do so like this:
+
+.. code-block:: python
+
+    db.index_foreign_keys()
+
 .. _python_api_hash:
 
 Setting an ID based on the hash of the row contents

sqlite_utils/cli.py

@@ -217,6 +217,20 @@ def add_foreign_key(path, table, column, other_table, other_column):
         raise click.ClickException(e)
 
 
+@cli.command(name="index-foreign-keys")
+@click.argument(
+    "path",
+    type=click.Path(exists=True, file_okay=True, dir_okay=False, allow_dash=False),
+    required=True,
+)
+def index_foreign_keys(path):
+    """
+    Ensure every foreign key column has an index on it.
+    """
+    db = sqlite_utils.Database(path)
+    db.index_foreign_keys()
+
+
 @cli.command(name="create-index")
 @click.argument(
     "path",

sqlite_utils/db.py

@@ -315,6 +315,16 @@ class Database:
         # can see the newly created foreign key.
         self.vacuum()
 
+    def index_foreign_keys(self):
+        for table_name in self.table_names():
+            table = self[table_name]
+            existing_indexes = {
+                i.columns[0] for i in table.indexes if len(i.columns) == 1
+            }
+            for fk in table.foreign_keys:
+                if fk.column not in existing_indexes:
+                    table.create_index([fk.column])
+
     def vacuum(self):
         self.conn.execute("VACUUM;")
 

tests/test_cli.py

@@ -310,6 +310,16 @@ def test_add_column_foreign_key(db_path):
     assert "table 'bobcats' does not exist" in str(result.exception)
 
 
+def test_index_foreign_keys(db_path):
+    test_add_column_foreign_key(db_path)
+    db = Database(db_path)
+    assert [] == db["books"].indexes
+    result = CliRunner().invoke(cli.cli, ["index-foreign-keys", db_path])
+    assert [["author_id"], ["author_name_ref"]] == [
+        i.columns for i in db["books"].indexes
+    ]
+
+
 def test_enable_fts(db_path):
     assert None == Database(db_path)["Gosh"].detect_fts()
     result = CliRunner().invoke(

tests/test_create.py

@@ -361,6 +361,16 @@ def test_add_foreign_key_guess_table(fresh_db):
     )
 
 
+def test_index_foreign_keys(fresh_db):
+    test_add_foreign_key_guess_table(fresh_db)
+    assert [] == fresh_db["dogs"].indexes
+    fresh_db.index_foreign_keys()
+    assert [["breed_id"]] == [i.columns for i in fresh_db["dogs"].indexes]
+    # Calling it a second time should do nothing
+    fresh_db.index_foreign_keys()
+    assert [["breed_id"]] == [i.columns for i in fresh_db["dogs"].indexes]
+
+
 @pytest.mark.parametrize(
     "extra_data,expected_new_columns",
     [