feat: fix repeat to create casbin_sequence for PostgreSQL (#60)

Author: OutOfEastGate

src/main/java/org/casbin/adapter/JDBCBaseAdapter.java

@@ -127,7 +127,14 @@ protected void migrate() throws SQLException {
                 sql = renderActualSql("IF NOT EXISTS (SELECT * FROM sysobjects WHERE name='casbin_rule' and xtype='U') CREATE TABLE casbin_rule(id int NOT NULL primary key identity(1, 1), ptype VARCHAR(100) NOT NULL, v0 VARCHAR(100), v1 VARCHAR(100), v2 VARCHAR(100), v3 VARCHAR(100), v4 VARCHAR(100), v5 VARCHAR(100))");
                 break;
             case "PostgreSQL":
-                sql = renderActualSql("CREATE SEQUENCE IF NOT EXISTS CASBIN_SEQUENCE START 1;");
+                sql = renderActualSql("do $$ " +
+                        "BEGIN " +
+                        "IF (select count(*) from information_schema.tables where table_name = 'casbin_rule') = 0 " +
+                        "THEN " +
+                        "CREATE SEQUENCE IF NOT EXISTS CASBIN_SEQUENCE START 1; "+
+                        "END IF; " +
+                        "END; " +
+                        "$$;");
                 break;
             case "H2":
                 sql = renderActualSql("CREATE TABLE IF NOT EXISTS casbin_rule(id int GENERATED ALWAYS AS IDENTITY PRIMARY KEY, ptype VARCHAR(100) NOT NULL, v0 VARCHAR(100), v1 VARCHAR(100), v2 VARCHAR(100), v3 VARCHAR(100), v4 VARCHAR(100), v5 VARCHAR(100))");