Comment by dragonwriter

What's wrong is replacing a single query with a join with multiple round trips implementing part of the join logic in application code because you are using an RDBMS as if it were a fairly dumb repository of dusconnected tables.

  SELECT id FROM table1

Is fine, if what your app wants is the ids from table1. Its not good if it is just a prelude to:

  SELECT * FROM table2
  WHERE table1_id in (...ids from previous query...)

instead of just doing:

  SELECT table2.*
  FROM table1 
  INNER JOIN table2 ON (
    table2.table1_id == table1.id
  )