Database: academic
Question: return me the keyword, which have been contained by the most number of papers in VLDB conference .
SQL:
SELECT t1.keyword FROM publication_keyword AS t4 JOIN keyword AS t1 ON t4.kid  =  t1.kid JOIN publication AS t3 ON t3.pid  =  t4.pid JOIN conference AS t2 ON t3.cid  =  t2.cid WHERE t2.name  =  "VLDB" GROUP BY t1.keyword ORDER BY COUNT ( DISTINCT t3.title ) DESC LIMIT 1;