[PROPOSAL] : Disallow use of empty column name in (column_name '') in ALTER or CREATE of foreign table.

Hi,

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Actual column names used while creation of foreign table are not allowed to
be an
empty string, but when we use column_name as an empty string in OPTIONS
during
CREATE or ALTER of foreign tables, it is allowed.

*EXAMPLES:-*
1) CREATE FOREIGN TABLE test_fdw(*"" *VARCHAR(15), id VARCHAR(5)) SERVER
localhost_fdw OPTIONS (schema_name 'public', table_name 'test');
ERROR: zero-length delimited identifier at or near """"
LINE 1: CREATE FOREIGN TABLE test_fdw("" VARCHAR(15), id VARCHAR(5))...

2) CREATE FOREIGN TABLE test_fdw(name VARCHAR(15) *OPTIONS* *(column_name
'')*, id VARCHAR(5)) SERVER localhost_fdw OPTIONS (schema_name 'public',
table_name 'test');
CREATE FOREIGN TABLE

postgres(at)43832=#\d test_fdw
Foreign table "public.test_fdw"
Column | Type | Collation | Nullable | Default | FDW
options
--------+-----------------------+-----------+----------+---------+------------------
name | character varying(15) | | | |
*(column_name
'')*
id | character varying(5) | | | |
Server: localhost_fdw
FDW options: (schema_name 'public', table_name 'test')

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Due to the above, when we try to simply select a remote table, the remote
query uses
the empty column name from the FDW column option and the select fails.

*EXAMPLES:-*
1) select * from test_fdw;
ERROR: zero-length delimited identifier at or near """"
CONTEXT: remote SQL command: SELECT "", id FROM public.test

2) explain verbose select * from test_fdw;
QUERY PLAN
--------------------------------------------------------------------------
Foreign Scan on public.test_fdw (cost=100.00..297.66 rows=853 width=72)
Output: name, id
Remote SQL: SELECT "", id FROM public.test
(3 rows)

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We can fix this issue either during fetching of FDW column option names
while
building remote query or we do not allow at CREATE or ALTER of foreign
tables itself.
We think it would be better to disallow adding the column_name option as
empty in
CREATE or ALTER itself as we do not allow empty actual column names for a
foreign
table. Unless I missed to understand the purpose of allowing column_name as
empty.

*THE PROPOSED SOLUTION OUTPUT:-*
1) CREATE FOREIGN TABLE test_fdw(name VARCHAR(15) OPTIONS *(column_name '')*,
id VARCHAR(5)) SERVER localhost_fdw OPTIONS (schema_name 'public',
table_name 'test');
ERROR: column generic option name cannot be empty

2) CREATE FOREIGN TABLE test_fdw(name VARCHAR(15), id VARCHAR(5)) SERVER
localhost_fdw OPTIONS (schema_name 'public', table_name 'test');
CREATE FOREIGN TABLE

ALTER FOREIGN TABLE test_fdw ALTER COLUMN id OPTIONS *(column_name '')*;
ERROR: column generic option name cannot be empty

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PFA, the fix and test cases patches attached. I ran "make check world" and
do
not see any failure related to patches. But, I do see an existing failure
t/001_pgbench_with_server.pl

Regards,
Nishant.

P.S
Thanks to Jeevan Chalke and Suraj Kharage for their inputs for the proposal.