check if sequence exists before creation for postgresql

There is a problem when using sequences with PostgreSql, if the sequence already exists and we try to create a table (even with checkfirst=true) an error raises telling that the sequence already exists.

Index: lib/sqlalchemy/databases/postgres.py
===================================================================
--- lib/sqlalchemy/databases/postgres.py    (revision 1802)
+++ lib/sqlalchemy/databases/postgres.py    (working copy)
@@ -348,9 +348,14 @@

     def visit_sequence(self, sequence):
         if not sequence.optional:
-            self.append("CREATE SEQUENCE %s" % sequence.name)
-            self.execute()
-            
+            r = self.connection.execute(**SELECT relname FROM pg_class WHERE relkind = 'S'
+    AND relnamespace IN ( SELECT oid FROM pg_namespace WHERE nspname NOT LIKE 'pg_%%'
+    AND nspname != 'information_schema'
+    AND relname = %(seqname)s);**, {'seqname': sequence.name})
+            if r.rowcount == 0:
+                self.append("CREATE SEQUENCE %s" % sequence.name)
+                self.execute()
+
 class PGSchemaDropper(ansisql.ANSISchemaDropper):
     def visit_sequence(self, sequence):
         if not sequence.optional:

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