Github user dongjoon-hyun commented on the issue:

    https://github.com/apache/spark/pull/19911
  
    @bolkedebruin , @srowen , @gatorsmile .
    
    PostgreSQL has `inheritance` between tables. Due to the following side 
effects, I'm not sure about this PR. Any thoughts?
    ```sql
    postgres=# CREATE TABLE parent(a INT);
    CREATE TABLE
    postgres=# CREATE TABLE child(b INT) INHERITS (parent);
    CREATE TABLE
    postgres=# INSERT INTO parent VALUES(1);
    INSERT 0 1
    postgres=# SELECT * FROM parent;
     a
    ---
     1
    (1 row)
    
    postgres=# INSERT INTO child VALUES(2);
    INSERT 0 1
    postgres=# SELECT * FROM parent;
     a
    ---
     1
     2
    (2 rows)
    
    postgres=# SELECT * FROM child;
     a | b
    ---+---
     2 |
    (1 row)
    
    postgres=# TRUNCATE TABLE parent;
    TRUNCATE TABLE
    postgres=# SELECT * FROM parent;
     a
    ---
    (0 rows)
    
    postgres=# SELECT * FROM child;
     a | b
    ---+---
    (0 rows)
    ```


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