improve the performance of flink sql job which lookup 40+ table

[snack white]

HI:
      My  flink  version is 1.10  use per-job mode , my   sql like 

```
select
  column1, t2.xx2, t3.xx3,t4.xx4
  …  t40.xx40
from
  main_table 
  left join lookup_1 FOR SYSTEM_TIME AS OF t1.proc_time AS t2 on t1.xx= t2.xx
  left join lookup_2 FOR SYSTEM_TIME AS OF t1.proc_time AS t3 on t1.xx= t3.xx
  left join lookup_3 FOR SYSTEM_TIME AS OF t1.proc_time AS t4 on t1.xx= t4.xx
  left join lookup_4 FOR SYSTEM_TIME AS OF t1.proc_time AS t5 on t1.xx= t5.xx
  left join lookup_5 FOR SYSTEM_TIME AS OF t1.proc_time AS t6 on t1.xx= t6.xx
  left join lookup_6 FOR SYSTEM_TIME AS OF t1.proc_time AS t7 on t1.xx= t7.xx
...

  left join lookup_40 FOR SYSTEM_TIME AS OF t1.proc_time AS t40 on t1.xx= t40.xx
```

I have developed the async lookup feature , but that is not enough， maybe the 
current look up table is serial not parallelism ?  

Now I need help about how can I  improve the performance of my sql job .

Best 
White