fix statements in on run start hooks (#693)

* fix statements in on run start hooks

* do nothing on empty hook

* pr feedback

dbt/node_runners.py

@@ -14,7 +14,6 @@
 import dbt.templates
 import dbt.writer
 
-import os
 import time
 
 
@@ -292,41 +291,48 @@ def run_hooks(cls, project, adapter, flat_graph, hook_type):
         nodes = flat_graph.get('nodes', {}).values()
         hooks = get_nodes_by_tags(nodes, {hook_type}, NodeType.Operation)
 
-        # This will clear out an open transaction if there is one.
-        # on-run-* hooks should run outside of a transaction. This happens b/c
-        # psycopg2 automatically begins a transaction when a connection is
-        # created. TODO : Move transaction logic out of here, and implement
-        # a for-loop over these sql statements in jinja-land. Also, consider
-        # configuring psycopg2 (and other adapters?) to ensure that a
-        # transaction is only created if dbt initiates it.
-        conn_name = adapter.clear_transaction(profile)
-
         compiled_hooks = []
         for hook in hooks:
+            model_name = hook.get('name')
+
+            # This will clear out an open transaction if there is one.
+            # on-run-* hooks should run outside of a transaction. This happens
+            # b/c psycopg2 automatically begins a transaction when a connection
+            # is created. TODO : Move transaction logic out of here, and
+            # implement a for-loop over these sql statements in jinja-land.
+            # Also, consider configuring psycopg2 (and other adapters?) to
+            # ensure that a transaction is only created if dbt initiates it.
+            adapter.clear_transaction(profile, model_name)
             compiled = cls._compile_node(adapter, project, hook, flat_graph)
-            model_name = compiled.get('name')
             statement = compiled['wrapped_sql']
 
             hook_index = hook.get('index', len(hooks))
             hook_dict = dbt.hooks.get_hook_dict(statement, index=hook_index)
             compiled_hooks.append(hook_dict)
+            adapter.release_connection(profile, model_name)
 
         ordered_hooks = sorted(compiled_hooks, key=lambda h: h.get('index', 0))
 
         for hook in ordered_hooks:
+            model_name = hook.get('name')
+
             if dbt.flags.STRICT_MODE:
                 dbt.contracts.graph.parsed.validate_hook(hook)
 
             sql = hook.get('sql', '')
-            adapter.execute_one(profile, sql, model_name=conn_name,
-                                auto_begin=False)
-            adapter.release_connection(profile, conn_name)
+
+            if len(sql.strip()) > 0:
+                adapter.execute_one(profile, sql, model_name=model_name,
+                                    auto_begin=False)
+
+                adapter.release_connection(profile, model_name)
 
     @classmethod
     def safe_run_hooks(cls, project, adapter, flat_graph, hook_type):
         try:
             cls.run_hooks(project, adapter, flat_graph, hook_type)
-        except dbt.exceptions.RuntimeException as e:
+
+        except dbt.exceptions.RuntimeException:
             logger.info("Database error while running {}".format(hook_type))
             raise