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

2 files changed, 27 insertions, 0 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.