你可以用 union all 和窗口功能。您没有列出表列，所以我假设 col1 以及 col2 :

insert into tableX (col1, col2)
select col1, col2 from table1
union all 
select t2.col1, t2.col2
from (select t2.*, row_number() over(order by random()) from table2 t2) t2
inner join (select count(*) cnt from table1) t1 on t2.rn <= 10000 - t1.cnt

中的第一个查询 union all 从中选择所有行 table1 . 第二个查询将随机行号分配给 table2 ，然后根据需要选择尽可能多的行以达到 10000 .
实际上，从两个表中选择所有行可能更简单，然后 order by 以及 limit 在外部查询中：

insert into tableX (col1, col2)
select col1, col2
from (
    select col1, col2, 't1' which from table1
    union all 
    select col1, col2, 't2' from table2
) t
order by which, random()
limit 10000

with inparms as (
  select 10000 as target_rows
), acount as (
  select count(*) as acount, inparms.target_rows 
    from tablea
   cross join inparms
), btag as (
  select b.*, 'tableb' as tabsource, 
         row_number() over (order by random()) as rnum
    from tableb
)
select a.*, 'tablea', row_number() over (order by 1) as rnum
  from tablea
union all
select b.*
  from btag b
  join acount a on b.rnum <= a.target_rows - a.acount
;