Ontime/SSB example datasets create table fields with not null

[sundy-li]

I think it's better to define the table schema as not null as possible.
This can show better performance in all queries.

https://github.com/datafuselabs/databend/blob/a54a4632f44c329d42ac71bfa7c0740034a00b8c/cli/src/cmds/generates/ddls/ontime.sql

cc @ZhiHanZ @wubx

[Xuanwo]

I thought we are not null by default? Based on discussion over #4234

[sundy-li]

Based on that discussion, it's null by default..

[Xuanwo]

Oh, must be my fault...

[wubx]

Can we use :

Year                            UInt16 not null default '',

replace

Year                            UInt16,

[sundy-li]

Year UInt16 not null defaults to 0.
C String not null defaults to ''.

You can change the default expression, but please don't use default '' for primitive columns.