server: sample DB for missing indexes, should prevent duplication from separate threads

server/sql/need-search-summary.sql

@@ -3,6 +3,6 @@ SELECT
     link,
     clean_summary
 FROM
-    post AS p
+    post AS p TABLESAMPLE SYSTEM (.1)
 INNER JOIN feed AS f ON p.site = f.slug -- necessary to weed out nzb posts
 WHERE search_summary IS NULL;