Adding a primary key to table in a fault-tolerant manner (PostgreSQL)

Add_PK_pgsql.sql

DO $$
    DECLARE __mytext character varying(200);
BEGIN
    __mytext := 'Test message';
    --RAISE NOTICE __mytext;
    RAISE NOTICE '%', __mytext;
    RAISE NOTICE '% %', 'arg1', 'arg2';
    
    
    IF 0 < 
    (
        SELECT COUNT(*) FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS 
	WHERE CONSTRAINT_TYPE = 'PRIMARY KEY' 
	AND LOWER(TABLE_SCHEMA) = LOWER('public') 
	AND LOWER(TABLE_NAME) = LOWER('T_SYS_ApertureColorToHex') 
	AND LOWER(CONSTRAINT_NAME) = LOWER('PK_T_SYS_ApertureColorToHex') 
    ) THEN 
        RAISE NOTICE 'Dropping primary-key PK_T_SYS_ApertureColorToHex'; 
	ALTER TABLE T_SYS_ApertureColorToHex DROP CONSTRAINT PK_T_SYS_ApertureColorToHex;
    END IF;
    
    
    IF 0 =  
    (
	SELECT COUNT(*) FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS 
	WHERE CONSTRAINT_TYPE = 'PRIMARY KEY' 
	AND TABLE_SCHEMA = 'public' 
	AND LOWER(TABLE_NAME) = LOWER('T_SYS_ApertureColorToHex') 
	-- AND LOWER(CONSTRAINT_NAME) = LOWER('PK_T_SYS_ApertureColorToHex') 
    ) THEN 
        RAISE NOTICE 'Creating primary-key PK_T_SYS_ApertureColorToHex'; 
        ALTER TABLE T_SYS_ApertureColorToHex ADD CONSTRAINT PK_T_SYS_ApertureColorToHex PRIMARY KEY (COL_Aperture); 
    END IF;
    
END$$;