Setting the comment of a column to that of another column in Postgresql

Question

Suppose I create a table in Postgresql with a comment on a column:

create table t1 (
   c1 varchar(10)
);
comment on column t1.c1 is \'foo\';

Answer 1

You can retrieve comments on columns using the system function col_description(table_oid, column_number). See this page for further details.

Answer 2

The next thing to know is how to obtain the table oid. I think that using this as part of comment on will not work, as you suspect.

    postgres=# create table comtest1 (id int, val varchar);
    CREATE TABLE
    postgres=# insert into comtest1 values (1,'a');
    INSERT 0 1
    postgres=# select distinct tableoid from comtest1;
     tableoid
    ----------
        32792
    (1 row)

    postgres=# comment on column comtest1.id is 'Identifier Number One';
    COMMENT
    postgres=# select col_description(32792,1);
        col_description
    -----------------------
     Identifier Number One
    (1 row)

Anyhow, I whipped up a quick plpgsql function to copy comments from one table/column pair to another. You have to createlang plpgsql on the database and use it like this:

    Copy the comment on the first column of table comtest1 to the id 
    column of the table comtest2. Yes, it should be improved but 
    that's left as work for the reader.

    postgres=# select copy_comment('comtest1',1,'comtest2','id');
     copy_comment
    --------------
                1
    (1 row)

CREATE OR REPLACE FUNCTION copy_comment(varchar,int,varchar,varchar) RETURNS int AS $PROC$
DECLARE
        src_tbl ALIAS FOR $1;
        src_col ALIAS FOR $2;
        dst_tbl ALIAS FOR $3;
        dst_col ALIAS FOR $4;
        row RECORD;
        oid INT;
        comment VARCHAR;
BEGIN
        FOR row IN EXECUTE 'SELECT DISTINCT tableoid FROM ' || quote_ident(src_tbl) LOOP
                oid := row.tableoid;
        END LOOP;

        FOR row IN EXECUTE 'SELECT col_description(' || quote_literal(oid) || ',' || quote_literal(src_col) || ')' LOOP
                comment := row.col_description;
        END LOOP;

        EXECUTE 'COMMENT ON COLUMN ' || quote_ident(dst_tbl) || '.' || quote_ident(dst_col) || ' IS ' || quote_literal(comment);

        RETURN 1;
END;
$PROC$ LANGUAGE plpgsql;