Question

How to raise error from PostgreSQL SQL statement if some condition is met?
I tried code below but got error.

CREATE OR REPLACE FUNCTION "exec"(text)
  RETURNS text AS
$BODY$ 
    BEGIN 
      EXECUTE $1; 
      RETURN $1; 
    END; 
$BODY$
  LANGUAGE plpgsql VOLATILE;

-- ERROR:  syntax error at or near "raise"
-- LINE 1: raise 'test' 

SELECT exec('raise ''test'' ') WHERE TRUE

In real application TRUE is replaced by some condition.

Update

I tried to extend answer to pass exception message parameters.
Tried code below but got syntax error.
How to pass message parameters ?

CREATE OR REPLACE FUNCTION exec(text, variadic ) 
  RETURNS void LANGUAGE plpgsql AS 
$BODY$  
BEGIN  
   RAISE EXCEPTION  $1, $2;  
END;  
$BODY$; 

SELECT exec('Exception Param1=% Param2=%', 'param1', 2 ); 

Answer 1

You cannot call RAISE dynamically (with EXECUTE) in PL/pgSQL – that only works for SQL statements, and RAISE is a PL/pgSQL command.

Use this simple function instead:

CREATE OR REPLACE FUNCTION f_raise(text)
  RETURNS void
  LANGUAGE plpgsql AS
$func$
BEGIN
   RAISE EXCEPTION '%', $1;
END
$func$;

Call:

SELECT f_raise('My message is empty!');

Related:

• Generate an exception with a Context

Additional answer to comment

CREATE OR REPLACE FUNCTION f_raise1(VARIADIC text[])
  RETURNS void
  LANGUAGE plpgsql AS
$func$
BEGIN 
   RAISE EXCEPTION 'Reading % % %!', $1[1], $1[2], $1[3];
END
$func$;

Call:

SELECT f_raise1('the','manual','educates');

• VARIADIC is not a data type, but an argument mode.

• Elements have to be handled like any other array element.

• To use multiple variables in a RAISE statement, put multiple % into the message text.

The above example will fail if no $3 is passed. You’d have to assemble a string from the variable number of input elements. Example:

CREATE OR REPLACE FUNCTION f_raise2(VARIADIC _arr text[]) 
  RETURNS void
  LANGUAGE plpgsql AS 
$func$  
DECLARE
   _msg text := array_to_string(_arr, ' and ');  -- simple string construction
BEGIN  
   RAISE EXCEPTION 'Reading %!', _msg;
END
$func$;

Call:

SELECT f_raise2('the','manual','educates');

I doubt you need a VARIADIC parameter for this at all. Read the manual here.
Instead, define all parameters, maybe add defaults:

CREATE OR REPLACE FUNCTION f_raise3(_param1 text = ''
                                  , _param2 text = ''
                                  , _param3 text = 'educates')
  RETURNS void
  LANGUAGE plpgsql AS
$func$
BEGIN
   RAISE EXCEPTION 'Reading % % %!', $1, $2, $3;
END 
$func$;

Call:

SELECT f_raise3('the','manual','educates');

Or:

SELECT f_raise3();  -- defaults kick in