Implement checkfirst for Index.create(), Index.drop()

The :meth:`.Index.create` and :meth:`.Index.drop` methods now have a
parameter :paramref:`.Index.create.checkfirst`, in the same way as that of
:class:`.Table` and :class:`.Sequence`, which when enabled will cause the
operation to detect if the index exists (or not) before performing a create
or drop operation.

Fixes: #527
Change-Id: Idf994bc016359d0ae86cc64ccb20378115cb66d6

5 files changed, 126 insertions, 8 deletions

diff --git a/lib/sqlalchemy/engine/default.py b/lib/sqlalchemy/engine/default.py
index 9d457b800..fb1728eab 100644
--- a/lib/sqlalchemy/engine/default.py
+++ b/lib/sqlalchemy/engine/default.py
@@ -426,6 +426,15 @@ class DefaultDialect(interfaces.Dialect):
             )
         }
 
+    def has_index(self, connection, table_name, index_name, schema=None):
+        if not self.has_table(connection, table_name, schema=schema):
+            return False
+        for idx in self.get_indexes(connection, table_name, schema=schema):
+            if idx["name"] == index_name:
+                return True
+        else:
+            return False
+
     def validate_identifier(self, ident):
         if len(ident) > self.max_identifier_length:
             raise exc.IdentifierError(
diff --git a/lib/sqlalchemy/engine/interfaces.py b/lib/sqlalchemy/engine/interfaces.py
index 5bd3b1d3e..fddc1712f 100644
--- a/lib/sqlalchemy/engine/interfaces.py
+++ b/lib/sqlalchemy/engine/interfaces.py
@@ -439,6 +439,24 @@ class Dialect(object):
 
         raise NotImplementedError()
 
+    def has_index(self, connection, table_name, index_name, schema=None):
+        """Check the existence of a particular index name in the database.
+
+        Given a :class:`.Connection` object, a string
+        `table_name` and stiring index name, return True if an index of the
+        given name on the given table exists, false otherwise.
+
+        The :class:`.DefaultDialect` implements this in terms of the
+        :meth:`.Dialect.has_table` and :meth:`.Dialect.get_indexes` methods,
+        however dialects can implement a more performant version.
+
+
+        .. versionadded:: 1.4
+
+        """
+
+        raise NotImplementedError()
+
     def has_sequence(self, connection, sequence_name, schema=None, **kw):
         """Check the existence of a particular sequence in the database.
 
diff --git a/lib/sqlalchemy/sql/ddl.py b/lib/sqlalchemy/sql/ddl.py
index ff36a68e4..ef6614b61 100644
--- a/lib/sqlalchemy/sql/ddl.py
+++ b/lib/sqlalchemy/sql/ddl.py
@@ -732,6 +732,17 @@ class SchemaGenerator(DDLBase):
             self.connection, table.name, schema=effective_schema
         )
 
+    def _can_create_index(self, index):
+        effective_schema = self.connection.schema_for_object(index.table)
+        if effective_schema:
+            self.dialect.validate_identifier(effective_schema)
+        return not self.checkfirst or not self.dialect.has_index(
+            self.connection,
+            index.table.name,
+            index.name,
+            schema=effective_schema,
+        )
+
     def _can_create_sequence(self, sequence):
         effective_schema = self.connection.schema_for_object(sequence)
 
@@ -831,7 +842,7 @@ class SchemaGenerator(DDLBase):
 
         if hasattr(table, "indexes"):
             for index in table.indexes:
-                self.traverse_single(index)
+                self.traverse_single(index, create_ok=True)
 
         if self.dialect.supports_comments and not self.dialect.inline_comments:
             if table.comment is not None:
@@ -859,7 +870,9 @@ class SchemaGenerator(DDLBase):
             return
         self.connection.execute(CreateSequence(sequence))
 
-    def visit_index(self, index):
+    def visit_index(self, index, create_ok=False):
+        if not create_ok and not self._can_create_index(index):
+            return
         self.connection.execute(CreateIndex(index))
 
 
@@ -973,6 +986,17 @@ class SchemaDropper(DDLBase):
             self.connection, table.name, schema=effective_schema
         )
 
+    def _can_drop_index(self, index):
+        effective_schema = self.connection.schema_for_object(index.table)
+        if effective_schema:
+            self.dialect.validate_identifier(effective_schema)
+        return not self.checkfirst or self.dialect.has_index(
+            self.connection,
+            index.table.name,
+            index.name,
+            schema=effective_schema,
+        )
+
     def _can_drop_sequence(self, sequence):
         effective_schema = self.connection.schema_for_object(sequence)
         return self.dialect.supports_sequences and (
@@ -985,7 +1009,10 @@ class SchemaDropper(DDLBase):
             )
         )
 
-    def visit_index(self, index):
+    def visit_index(self, index, drop_ok=False):
+        if not drop_ok and not self._can_drop_index(index):
+            return
+
         self.connection.execute(DropIndex(index))
 
     def visit_table(self, table, drop_ok=False, _is_metadata_operation=False):
diff --git a/lib/sqlalchemy/sql/schema.py b/lib/sqlalchemy/sql/schema.py
index 091012896..d9667e445 100644
--- a/lib/sqlalchemy/sql/schema.py
+++ b/lib/sqlalchemy/sql/schema.py
@@ -3670,7 +3670,7 @@ class Index(DialectKWArgs, ColumnCollectionMixin, SchemaItem):
 
         return self.table.bind
 
-    def create(self, bind=None):
+    def create(self, bind=None, checkfirst=False):
         """Issue a ``CREATE`` statement for this
         :class:`.Index`, using the given :class:`.Connectable`
         for connectivity.
@@ -3682,10 +3682,10 @@ class Index(DialectKWArgs, ColumnCollectionMixin, SchemaItem):
         """
         if bind is None:
             bind = _bind_or_error(self)
-        bind._run_ddl_visitor(ddl.SchemaGenerator, self)
+        bind._run_ddl_visitor(ddl.SchemaGenerator, self, checkfirst=checkfirst)
         return self
 
-    def drop(self, bind=None):
+    def drop(self, bind=None, checkfirst=False):
         """Issue a ``DROP`` statement for this
         :class:`.Index`, using the given :class:`.Connectable`
         for connectivity.
@@ -3697,7 +3697,7 @@ class Index(DialectKWArgs, ColumnCollectionMixin, SchemaItem):
         """
         if bind is None:
             bind = _bind_or_error(self)
-        bind._run_ddl_visitor(ddl.SchemaDropper, self)
+        bind._run_ddl_visitor(ddl.SchemaDropper, self, checkfirst=checkfirst)
 
     def __repr__(self):
         return "Index(%s)" % (
diff --git a/lib/sqlalchemy/testing/suite/test_reflection.py b/lib/sqlalchemy/testing/suite/test_reflection.py
index 9ca13ec4e..9b6cec10c 100644
--- a/lib/sqlalchemy/testing/suite/test_reflection.py
+++ b/lib/sqlalchemy/testing/suite/test_reflection.py
@@ -77,6 +77,65 @@ class HasTableTest(fixtures.TablesTest):
             )
 
 
+class HasIndexTest(fixtures.TablesTest):
+    __backend__ = True
+
+    @classmethod
+    def define_tables(cls, metadata):
+        tt = Table(
+            "test_table",
+            metadata,
+            Column("id", Integer, primary_key=True),
+            Column("data", String(50)),
+        )
+        Index("my_idx", tt.c.data)
+
+        if testing.requires.schemas.enabled:
+            tt = Table(
+                "test_table",
+                metadata,
+                Column("id", Integer, primary_key=True),
+                Column("data", String(50)),
+                schema=config.test_schema,
+            )
+            Index("my_idx_s", tt.c.data)
+
+    def test_has_index(self):
+        with config.db.begin() as conn:
+            assert config.db.dialect.has_index(conn, "test_table", "my_idx")
+            assert not config.db.dialect.has_index(
+                conn, "test_table", "my_idx_s"
+            )
+            assert not config.db.dialect.has_index(
+                conn, "nonexistent_table", "my_idx"
+            )
+            assert not config.db.dialect.has_index(
+                conn, "test_table", "nonexistent_idx"
+            )
+
+    @testing.requires.schemas
+    def test_has_index_schema(self):
+        with config.db.begin() as conn:
+            assert config.db.dialect.has_index(
+                conn, "test_table", "my_idx_s", schema=config.test_schema
+            )
+            assert not config.db.dialect.has_index(
+                conn, "test_table", "my_idx", schema=config.test_schema
+            )
+            assert not config.db.dialect.has_index(
+                conn,
+                "nonexistent_table",
+                "my_idx_s",
+                schema=config.test_schema,
+            )
+            assert not config.db.dialect.has_index(
+                conn,
+                "test_table",
+                "nonexistent_idx_s",
+                schema=config.test_schema,
+            )
+
+
 class ComponentReflectionTest(fixtures.TablesTest):
     run_inserts = run_deletes = None
 
@@ -1129,4 +1188,9 @@ class NormalizedNameTest(fixtures.TablesTest):
         eq_(tablenames[1].upper(), tablenames[1].lower())
 
 
-__all__ = ("ComponentReflectionTest", "HasTableTest", "NormalizedNameTest")
+__all__ = (
+    "ComponentReflectionTest",
+    "HasTableTest",
+    "HasIndexTest",
+    "NormalizedNameTest",
+)