negora <negora@xxxxxxxxxx> wrote: > I've a doubt about how the PostgreSQL planner makes a hash join. > Let's suppose that I've 2 tables, one of students and the other > one of parents in a many-to-one relation. I want to do something > like this: > > SELECT s.complete_name, f.complete_name > FROM students AS s > JOIN fathers AS f ON f.id_father = s.id_father; > > Using the ANALYZE command, I've checked that the planner firstly > scans and extracts the required information from "fathers", builds > a temporary hash table from it, then scans "students", and finally > joins the information from this table and the temporary one > employing the relation "f.id_father = s.id_father". This sort of plan is sometimes used when the optimizer expects the hash table to fit into RAM, based on statistics and your work_mem setting. If it does fit, that's one sequential scan of the father table's heap, and a hashed lookup into RAM to find the father to match each student. For the sort of query you're showing, that's typically a very good plan. -Kevin -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
