Variable substitution in PL/pgSQL

I have a select statement that is generated dynamically based on the supplied parameter. The problem is that postgresql always says:

argument of WHERE must be type boolean, not type character varying no matter what the parameter is. Did I miss anything?

CREATE OR REPLACE FUNCTION getuid(name character varying) RETURNS integer AS $$
    DECLARE 
        statement varchar;
        uid integer;
    BEGIN
        IF ($1 = '') THEN
            statement := 'TRUE'开发者_JAVA百科;
            statement := CAST(statement AS BOOLEAN);
        ELSE
            statement := 'users.keywords ILIKE''' || '%' || $1 || '%''';
        END IF;
        SELECT INTO uid id FROM users WHERE "statement";
        RETURN uid;
    END;
$$ LANGUAGE plpgsql

You need EXECUTE if you want to generate dynamic commands inside a function. You could also use two different sections:

CREATE OR REPLACE FUNCTION getuid(name character varying) RETURNS integer AS $$
    DECLARE 
        statement varchar;
        uid integer;
    BEGIN
        IF ($1 = '' OR $1 IS NULL) THEN -- section 1
            SELECT id INTO uid FROM users;
        ELSE -- section 2
            SELECT id INTO uid FROM users WHERE users.keywords ILIKE '%' || $1 || '%';
        END IF;

        RETURN uid;
    END;
$$ LANGUAGE plpgsql;

EXECUTE is a PL/pgSQL statement and not SQL statement. So you have to wrap your dynamic query into PL/pgSQL stored procedure.

Be careful about variable substitution and do not forget to use quote_literal() or quote_nullable() when building up you query.

Have look in documentation here: http://www.postgresql.org/docs/current/static/plpgsql-statements.html#PLPGSQL-STATEMENTS-EXECUTING-DYN