Re: Internal operations when the planner makes a hash join.

From: Kevin Grittner
Subject: Re: Internal operations when the planner makes a hash join.
Date: ,
Msg-id: 4B83A03E020000250002F4FD@gw.wicourts.gov
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In response to: Internal operations when the planner makes a hash join.  (negora)
Responses: Re: Internal operations when the planner makes a hash join.  (negora)
List: pgsql-performance


negora <> 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


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