Column names with line breaks - postgresql

Question

I know that for text strings in PostgreSQL line breaks are unified by appending symbol E or e in front of the text:
SELECT E'first\nsecond'
results in:
first
second
But PostgreSQL also support line breaks within column names - not sure why or how evil this practice is, but one can do the following:
CREATE TABLE One("first\nsecond" text);
CREATE TABLE Two("first
second" text);
When you are unfortunate enough to run into one of these, you would find that while these queries work:
SELECT "first\nsecond" from One;
SELECT "first
second" from Two;
these ones do not:
SELECT "first
second" from One;
SELECT "first\nsecond" from Two;
My question is: Is there a way in PostgreSQL that unifies such differences, similar to the situation with the column values?
I have tried putting E in front of "first\nsecond" column names, but it is not supported. Trying to put \r\n instead (I'm using Windows) gave me a third type of column names, one that can only be queried as:
SELECT "first\r\nsecond" FROM Third

Answer 1

Column names are identifiers, and the gory details of the syntax for identifiers are described at:
http://www.postgresql.org/docs/current/static/sql-syntax-lexical.html#SQL-SYNTAX-IDENTIFIERS
TL;DR: use the U&"..." syntax to inject non-printable characters into identifiers through their Unicode codepoints, and there's no way to unify CR,LF with LF alone.
How to refer to the column in a single line
We're allowed to use Unicode escape sequences in identifiers, so per documentation, the following does work:
select U&"first\000asecond" from Two;
if it's just a newline character between the two words.
What happens with the queries on the first table
The table is created with:
CREATE TABLE One("first\nsecond" text);
As the backslash character has no special meaning here, this column does not contain any newline.
It contains first followed by \ followed by n followed by second.
So:
SELECT "first\nsecond" from One;
does work because it's the same as what's in the CREATE TABLE
whereas
SELECT "first
second" from One;
fails because there's a newline in that SELECT where the actual column name in the table has a backslash followed by a n.
What happens with the queries on the second table
This is the opposite of "One".
CREATE TABLE Two("first
second" text);
The newline is taken verbatim and is part of the column.
So
SELECT "first
second" from Two;
works because the newline is there exactly as in the CREATE TABLE,
with an embedded newline,
whereas
SELECT "first\nsecond" from Two;
fails because as previously \n in this context does not mean a newline.
Carriage Return followed by Newline, or anything weirder
As mentioned in comments and your edit, this could be carriage return and newline instead, in which case the following should do:
select U&"first\000d\000asecond" from Two;
although in my test, hitting Enter in the middle of a column with psql on Unix and Windows has the same effect: a single newline in the column's name.
To check what exact characters ended up in a column name, we can inspect them in hexadecimal.
When applied to your create table example, from inside psql under Unix:
CREATE TABLE Two("first
second" text);
select convert_to(column_name::text,'UTF-8')
from information_schema.columns
where table_schema='public'
and table_name='two';
The result is:
convert_to
----------------------------
\x66697273740a7365636f6e64
For more complex cases (e.g. non-ascii characters with several bytes in UTF-8), a more advanced query might help, for easy-to-read codepoints:
select c,lpad(to_hex(ascii(c)),4,'0') from (
select regexp_split_to_table(column_name::text,'') as c
from information_schema.columns
where table_schema='public'
and table_name='two'
) as g;
c | lpad
---+------
f | 0066
i | 0069
r | 0072
s | 0073
t | 0074
+| 000a
|
s | 0073
e | 0065
c | 0063
o | 006f
n | 006e
d | 0064