diff --git a/phpBB/phpbb/db/driver/postgres.php b/phpBB/phpbb/db/driver/postgres.php
index 52a5b6b546..b004ecee30 100644
--- a/phpBB/phpbb/db/driver/postgres.php
+++ b/phpBB/phpbb/db/driver/postgres.php
@@ -277,9 +277,10 @@ class postgres extends \phpbb\db\driver\driver
 			$query_id = $this->query_result;
 		}
 
-		if ($cache && $cache->sql_exists($query_id))
+		$safe_query_id = $this->clean_query_id($query_id);
+		if ($cache && $cache->sql_exists($safe_query_id))
 		{
-			return $cache->sql_fetchrow($query_id);
+			return $cache->sql_fetchrow($safe_query_id);
 		}
 
 		return ($query_id) ? pg_fetch_assoc($query_id, null) : false;
@@ -297,14 +298,47 @@ class postgres extends \phpbb\db\driver\driver
 			$query_id = $this->query_result;
 		}
 
-		if ($cache && $cache->sql_exists($query_id))
+		$safe_query_id = $this->clean_query_id($query_id);
+		if ($cache && $cache->sql_exists($safe_query_id))
 		{
-			return $cache->sql_rowseek($rownum, $query_id);
+			return $cache->sql_rowseek($rownum, $safe_query_id);
 		}
 
 		return ($query_id) ? @pg_result_seek($query_id, $rownum) : false;
 	}
 
+	/**
+	 * {@inheritDoc}
+	 */
+	function sql_fetchfield($field, $rownum = false, $query_id = false)
+	{
+		global $cache;
+
+		if ($query_id === false)
+		{
+			$query_id = $this->query_result;
+		}
+
+		if ($query_id)
+		{
+			if ($rownum !== false)
+			{
+				$this->sql_rowseek($rownum, $query_id);
+			}
+
+			$safe_query_id = $this->clean_query_id($query_id);
+			if ($cache && !is_object($query_id) && $cache->sql_exists($safe_query_id))
+			{
+				return $cache->sql_fetchfield($safe_query_id, $field);
+			}
+
+			$row = $this->sql_fetchrow($query_id);
+			return (isset($row[$field])) ? $row[$field] : false;
+		}
+
+		return false;
+	}
+
 	/**
 	* {@inheritDoc}
 	*/
@@ -346,14 +380,15 @@ class postgres extends \phpbb\db\driver\driver
 			$query_id = $this->query_result;
 		}
 
-		if ($cache && !is_object($query_id) && $cache->sql_exists($query_id))
+		$safe_query_id = $this->clean_query_id($query_id);
+		if ($cache && !is_object($query_id) && $cache->sql_exists($safe_query_id))
 		{
-			return $cache->sql_freeresult($query_id);
+			return $cache->sql_freeresult($safe_query_id);
 		}
 
-		if (isset($this->open_queries[(int) $query_id]))
+		if (isset($this->open_queries[(int) $safe_query_id]))
 		{
-			unset($this->open_queries[(int) $query_id]);
+			unset($this->open_queries[(int) $safe_query_id]);
 			return pg_free_result($query_id);
 		}
 
@@ -505,4 +540,16 @@ class postgres extends \phpbb\db\driver\driver
 	{
 		return '"' . $msg . '"';
 	}
+
+	/**
+	 * Ensure query ID can be used by cache
+	 *
+	 * @param resource|int|string $query_id Mixed type query id
+	 *
+	 * @return int|string Query id in string or integer format
+	 */
+	private function clean_query_id($query_id)
+	{
+		return is_resource($query_id) ? (int) $query_id : $query_id;
+	}
 }