SELECT COUNT ( DISTINCT t2.title )  ,  t2.year FROM publication AS t2 JOIN conference AS t1 ON t2.cid  =  t1.cid WHERE t1.name  =  "VLDB" GROUP BY t2.year;
return me the number of papers published in the VLDB conference in each year .
[Schema (values) (types)]: | academic |  author : aid (text) , homepage (number) , name (text) , oid (text) | conference : cid (text) , homepage (number) , name (text) | domain : did (text) , name (number) | domain_author : aid (text) , did (number) | domain_conference : cid (text) , did (number) | journal : homepage (text) , jid (number) , name (text) | domain_journal : did (text) , jid (number) | keyword : keyword (text) , kid (number) | domain_keyword : did (text) , kid (number) | publication : abstract (text) , cid (number) , citation_num (text) , jid (text) , pid (number) , reference_num (number) , title (text) , year (text) | domain_publication : did (text) , pid (number) | organization : continent (text) , homepage (number) , name (text) , oid (text) | publication_keyword : pid (text) , kid (number) | writes : aid (text) , pid (number) | cite : cited (text) , citing (number);