When using COPY, there is an option called FORCE_NOT_NULL allowing to enforce a string to be not null even if it is not quoted. Here is an example of how it works:
=# CREATE TABLE aa (a text);
CREATE TABLE
=# \COPY aa FROM STDIN WITH (FORMAT csv, FORCE_NOT_NULL(a));
Enter data to be copied followed by a newline.
End with a backslash and a period on a line by itself.
>> ""
>>
>> \.
=# \pset null 'null'
Null display (null) is "null".
=# SELECT * FROM aa;
a
---
(2 rows)
In this case, even if the values are empty, FORCE_NOT_NULL forces them to be inserted as empty strings and not NULL values. Now, let's look at the new option FORCE_NULL that is introduced in Postgres 9.4 by this commit, feature actually written by Ian, hacker living in Japan as well as the author of this blog.
commit 3b5e03dca2afea7a2c12dbc8605175d0568b5555
Author: Andrew Dunstan <andrew@dunslane.net>
Date: Tue Mar 4 17:31:59 2014 -0500
Provide a FORCE NULL option to COPY in CSV mode.
This forces an input field containing the quoted null string to be
returned as a NULL. Without this option, only unquoted null strings
behave this way. This helps where some CSV producers insist on quoting
every field, whether or not it is needed. The option takes a list of
fields, and only applies to those columns. There is an equivalent
column-level option added to file_fdw.
Ian Barwick, with some tweaking by Andrew Dunstan, reviewed by Payal
Singh.
Contrary to FORCE_NOT_NULL, this can be used to force an empty value to be inserted as NULL even if it is quoted.
=# TRUNCATE aa;
TRUNCATE TABLE
=# \COPY aa FROM STDIN WITH (FORMAT csv, FORCE_NULL(a));
Enter data to be copied followed by a newline.
End with a backslash and a period on a line by itself.
>> ""
>>
>> \.
=# SELECT * FROM aa;
a
------
null
null
(2 rows)
file_fdw comes up as well with a similar new option, and it can be used on columns.
=# CREATE EXTENSION file_fdw;
CREATE EXTENSION
=# CREATE SERVER file_server FOREIGN DATA WRAPPER file_fdw;
CREATE SERVER
=# CREATE FOREIGN TABLE test (
a int,
b text OPTIONS (force_null 'true'),
c text OPTIONS (force_not_null 'true')
) SERVER file_server
OPTIONS ( filename '/path/to/data/test.csv', format 'csv');
CREATE FOREIGN TABLE
=# \! cat /path/to/data/test.csv
1,"",
2,,""
=# SELECT * FROM test;
a | b | c
---+------+---
1 | null |
2 | null |
(2 rows)
Note that both force_null and force_not_null cannot be specified on the same column.
=# ALTER FOREIGN TABLE test ALTER COLUMN b
OPTIONS (force_not_null 'true');
ERROR: 42601: conflicting or redundant options
HINT: option "force_not_null" cannot be used together with "force_null"
LOCATION: file_fdw_validator, file_fdw.c:274
And that's all for this short but instructive post, for a feature proved to be useful for Or**le to Postgres migrations.