测试版本：1.30.9 2021.05.17
注意：dfs表的delete、update从1.30.6版本才开始支持
内存表的删除方法：
t = table(1 2 3 as id, 4 NULL 6 as v1, 7..9 as v2) delete from t where isNull(v1)
dfs表的删除方法：
if(existsDatabase("dfs://hashdb")){
dropDatabase("dfs://hashdb")
}
db = database("dfs://hashdb", HASH, [INT, 2])
t = table(1 2 3 as id, 4 NULL 6 as v1, 7..9 as v2)
pt = db.createPartitionedTable(t, `pt, `id)
pt.append!(t)
delete from loadTable("dfs://hashdb", `pt) where isNull(v1)