Database: academic
Question: return me the number of keywords in VLDB conference .
SQL:
SELECT COUNT ( DISTINCT 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";