PostgreSQL: ERROR operator does not exist: || character varying - postgresql

Question

I need to check the condition within function using string_agg() function and need to assign it to variable. After assigning I need to execute the variable with value.
Example:
create or replace function funct1(a int,b varchar)
returns void as
$$
declare
wrclause varchar := '';
sqlq varchar ;
t varchar;
begin
IF (b IS NOT NULL ) THEN
wrclause := 'AND b IN ('|| b || ')';
END IF;
sqlq := string_agg('select *, abcd as "D" from ' ||table_namess,' Union all ') as namess
from tablescollection2 ud
inner join INFORMATION_SCHEMA.Tables so on ud.table_namess = so.Table_name
WHERE cola NOT IN (SELECT cola FROM tablet WHERE colb = || a ||) || wrclause; /* Error occurred here at = || a */
raise info '%',sqlq;
execute sqlq into t;
raise info '%',t;
end;
$$
language plpgsql;
Calling Function:
select funct1(1,'1,2,3');
Error:
ERROR: operator does not exist: || integer

Answer 1

|| is an operator for catenating two pieces of text, it requires you to have text (or something convertible to text) both before and after the operator, like so:
select 'a' || 'b'
select 'a' || 3
So while these seem to be valid:
wrclause := 'AND b IN ('|| b || ')';
sqlq := string_agg('select *, abcd as "D" from ' ||table_namess,' Union all ') as namess
This is definitely not:
WHERE cola NOT IN (SELECT cola FROM tablet WHERE colb = || a ||) || wrclause;
What were you trying to achieve here?
It looks like you may be trying to construct a query dynamically. You need to remember that you cannot mix free text with SQL and expect Postgres to sort it out, no programming or query language does that.
If that's your intention, you should construct the query string first in its entirety (in a variable), and then call EXECUTE with it to have it interpreted.
Have a look at these:
Postgres Dynamic Query Function
PostgreSQL - dynamic value as table name

Answer 2

This piece contains the syntax error
... IN (SELECT cola FROM tablet WHERE colb = || a ||) || ...
PostgreSQL can understand this, but will try to search for unary prefix (and a postfix) || operator, which are not exist by default (they can be created however, but the error message says, that's not the case)
Edit:
F.ex. these are valid (predefined) unary operators on numbers:
SELECT |/ 25.0, -- prefix, square root, result: 5.0
5 !, -- postfix, factorial, result: 120,
# -5, -- prefix, absolute, result: 5
# -5 !; -- mixed, result: 120