return me the number of papers on VLDB conference .
SELECT COUNT ( DISTINCT t2.title ) FROM publication AS t2 JOIN conference AS t1 ON t2.cid  =  t1.cid WHERE t1.name  =  "VLDB";
SELECT COUNT ( DISTINCT t2.title ) FROM publication AS t2 JOIN conference AS t1 ON t2.cid  =  t1.cid WHERE t1.name  =  "VLDB";
[{'COUNT ( DISTINCT t2.title )': 0}]