Basics fails; SELECT, UPDATE

Hello! Nice strangers. I'll keep to the basics, both (re)questing (?)
(pardon my english) and answering.

PostgreSQL won't select a specific row and it won't change an old value to
a new one. Not for me. Why the heck is that?

The funny thing is that in small tables I make to test things, in them
everything works!

These are the commands that won't work for me:

SELECT * FROM RealTable WHERE RealPrimaryKey = 'realvalue';

SELECT RealColumn FROM RealTable WHERE RealPrimaryKey = 'realvalue';

I get a blank sheet. The names of the columns are on top, otherways it's
blank. Why on earth????

It works without the ”WHERE”-part; I can see the full, filled table if I
want to. But not any specific row or rows.

Neither will this work, not if there allready is a value in ”RealColumn”:

UPDATE RealTable SET RealColumn = 'newrealvalue' WHERE RealPrimaryKey =
'realvalue';

Postgre SQL responds: ”Query returned successfully: 0 rows affected, 15 ms
execution time.”

Or other execution time, that differs; if it where allways the same,
something would be wrong.

I can put in

”

GRANT ALL ON RealTable TO public;

”

or not, the result is the same either way; the problems remains.

But in small tables like this, that I make to test things, in them
everything works!

CREATE TABLE T (T CHAR(2) PRIMARY KEY, Q CHAR(2));

INSERT INTO T (T, Q) VALUES ('A1', 'A2'), ('B1', 'B2');

SELECT * FROM T WHERE T = 'A1';

ALTER TABLE T ADD COLUMN R CHAR(2);

SELECT * FROM T WHERE T = 'A1';

UPDATE T SET R = 'C1' WHERE T = 'A1'

SELECT * FROM T WHERE T = 'A1';

UPDATE T SET Q = 'X' WHERE T = 'A1'

SELECT * FROM T WHERE T = 'A1';

Can anyone help me out?