PostGIS: ST_Within returns "t"/"f" letters instead of "true"/"false" booleans

Question

I have simple query which contains among others:

SELECT ST_Within(ST_GeomFromText('POINT(lon lat)', 4326), buildings.geom) AS is_within

So I expect the result column is_within to be Boolean value. And this is what happening when I test this on QGIS. Then I integrate it within some java code using the well known standard connection with driver class (nothing unusual):

Class.forName(DB_CLASS);
Connection connection = DriverManager.getConnection(DB_ADDRESS, DB_USER_NAME, DB_PASSWORD);
if (connection != null) {
    Statement statement = connection.createStatement();
    String SQL = ...;
    ResultSet resultSet = statement.executeQuery(SQL);
    while (resultSet.next()) {
        System.out.println(resultSet.getString(1));  //result is letters and not booleans
    }
    resultSet.close();
    statement.close();
    connection.close(); 
}

Now from some reason the result is "t" instead of "true" and "f" instead of "false". Why is this happening?

Answer 1

If you change the line

System.out.println(resultSet.getString(1));

to

System.out.println(resultSet.getBoolean(1));

you will get back a boolean value instead of a string.

Answer 2

The t/f vs. true/false is just a display issue. It is a boolean internally in PostgreSQL but depending on what driver you are using a boolean value takes on different representation with t/f being the standard canonical form for psql, pgAdmin and QGIS. In MSAccess it becomes -1.

In SQL ST_Within(..) = true vs. ST_Within = t vs. ST_Within all mean the same thing.

If you do this for example in pgAdmin,

SELECT true;

you get t

But you can cast a boolean to text and then it shows true/false

SELECT ST_Within('LINESTRING(1 2, 3 4, 2 4)'::geometry, 
    'POINT(3 4)'::geometry)::text;

Outputs:

false

Why SELECT t; is not considered boolean is a mystery to me :) Confused now :)