Github user gatorsmile commented on the pull request:
https://github.com/apache/spark/pull/10689#issuecomment-170619409
Give two tables `tbl_a` and `tbl_b`, `tbl_a` has **billions** of rows but
`tbl_b` has **thousands** of rows. `tbl_a` has one column `col_frkey_tbl_a`
whose values should be from `tbl_b`'s column `col_key_tbl_b`. However, one user
wants to do a quick check to confirm it. The query he can try is
```
select col_frkey_tbl_a from db.tbl_a limit 10000
intersect
select col_key_tbl_b from db.tbl_b
```
The above query can avoid fetching billions of rows from `tbl_a`.
Hopefully, it can answer your question. @hvanhovell
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