Postgres 9.5

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Help with an interesting Postgres question: Why isn't an Index Only Scan used on a partition accessed via the parent table?.

create table library(id int, name varchar, owner varchar);
create table books (id varchar, title varchar, library int);

insert into library values(1,'ABC','A'),(2,'DEF','B'),(3,'GHI','C');
insert into books values('a','xxx',1),('b','yyy',1),('c','zzz',2);
3 rows affected

3 rows affected

select 
    l.name, 
    array_agg(b.title) 
from library l left join books b on l.id=b.library
where l.id=3
group by l.name


name
array_agg


GHI
{NULL}


SELECT library.name, array_agg(b.title) AS book_list FROM library, 
(SELECT title FROM books WHERE books.library = :library_no) as b 
WHERE library.id = :library_no GROUP BY library.id
ERROR:  syntax error at or near ":"
LINE 2: (SELECT title FROM books WHERE books.library = :library_no) ...
                                                       ^