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