fix defautl key value for scans

Author: imranzaheer612

README.md

@@ -104,7 +104,7 @@ Usage
   Specifies which column should be treated as the unique row identifier.
   Usually set to key.
 
-- **prefix** as *string*, optional, default `/`
+- **prefix** as *string*, optional, default ``
 
   Restrict the scan to keys beginning with this prefix.
   If not provided, the FDW will fetch all keys from the etcd server

src/lib.rs

@@ -275,7 +275,8 @@ impl ForeignDataWrapper<EtcdFdwError> for EtcdFdw {
         self.fetch_key = colnames.contains(&String::from("key"));
         self.fetch_value = colnames.contains(&String::from("value"));
 
-        let key = prefix.clone().unwrap_or_else(|| String::from("/"));
+        // samllest possible valid key '\0', empty string will not work with with_range
+        let key = prefix.clone().unwrap_or_else(|| String::from("\0"));
         let result = self
             .rt
             .block_on(self.client.get(key, Some(get_options)));