-- ... stuff before
dynamic_select_cols := 'tbl1.foo, tbl1.bar, null AS theAlias1, null AS theAlias2, tbl2.foo2';
-- ... stuff after combining the dynamic queries
EXECUTE my_dynamic_query;

In my case the result of the query will be inserted in a temporary table in memory. The destination column for theAlias1 is of type INTEGER. When executing the query PostgreSQL gives me an error:

column "theAlias1" is of type integer but expression is of type text

So there is something wrong when the query is parsed, evaluated and executed.

Solving this was easy with the use of NULLIF. Zero equals zero so NULL is returned. Just as expected.

-- ... stuff before
dynamic_select_cols := 'tbl1.foo, tbl1.bar, NULLIF(0, 0) AS theAlias1, NULLIF(0, 0) AS theAlias2, tbl2.foo2';
-- ... stuff after combining the dynamic queries
EXECUTE my_dynamic_query;

Value too long for type character varying(64)

Within a function (written with plpgsql) a dynamic query is built and executed via sql-EXECUTE. The error PostgreSQL gives me stated out that the error was in the line containing the EXECUTE-statement. But the query was of type TEXT so there is no length limitation.

Guide me

In the end it showed that the error PostgreSQL gave me was misleading. The final dynamic query itself was fine. So the sql-EXECUTE cannot be the cause of the error. 

The function returns a custom (composite) type (built via PostgreSQL CREATE TYPE). This custom type consists of a few “columns”. One of this column was indeed of type character varying(64) and was simply to short to hold the data collected inside the function.

Fixing the data type of that column in the custom composite type fixes the error.