Left join vs Multiple selects?

Question

PostgreSQL

Which is better in terms of performance? versus
select a left join b on b.id = a.b_id

select from a // (получили 1,2,3)
select from b where id = 1,2,3

Answer 1

one). Your "SQL" is more like pseudocode
2). These queries are not equivalent (read more about LEFT JOIN)
3). The first option is faster (under reasonable assumptions), the second is a well-known anti-pattern used by novice programmers who are frightened by "slow joins". In fact, joins are exactly what relational databases can do very quickly and very advanced.

Answer 2

join if there are indices

Answer 3

I don't know about you, of course. But when I tested it on my database, it processes a request like:
select from b where id in (select id from a)