Re: LEFT JOIN ...

Morning ...

I'm trying to wrack my brain over something here, and no matter
how I try and look at it, I'm drawing a blank ...

I have two tables that are dependent on each other:

notes (86736 tuples) and note_links (173473 tuples)

The relationship is that one note can have several 'ppl' link'd to
it ...

I have a third table: calendar (11014 tuples) ... those calendar
entries link to a note.

So you have something like:

personA ---
personB --|--> note_links --> notes --[maybe]--> calendar entry
personC ---

now, the query I'm workign with is:

SELECT n.note, n.nid, n.type, c.act_type, c.status, nl.contact_lvl,
CASE WHEN c.act_start IS NULL
THEN date_part('epoch', n.added)
ELSE date_part('epoch', c.act_start)
END AS start
FROM note_links nl, notes n LEFT JOIN calendar c ON (n.nid = c.nid)
WHERE (n.type = 'A' OR n.type = 'N' OR n.type = 'H' OR n.type = 'C')
AND (nl.id = 15748 AND contact_lvl = 'company')
AND n.nid = nl.nid
ORDER BY start DESC;

Which explains out as:

NOTICE: QUERY PLAN:

Sort (cost=7446.32..7446.32 rows=1 width=88)
-> Nested Loop (cost=306.52..7446.31 rows=1 width=88)
-> Index Scan using note_links_id on note_links nl (cost=0.00..3.49 rows=1 width=16)
-> Materialize (cost=6692.63..6692.63 rows=60015 width=72)
-> Hash Join (cost=306.52..6692.63 rows=60015 width=72)
-> Seq Scan on notes n (cost=0.00..2903.98 rows=60015 width=36)
-> Hash (cost=206.22..206.22 rows=10122 width=36)
-> Seq Scan on calendar c (cost=0.00..206.22 rows=10122 width=36)

EXPLAIN

and takes forever to run ...

Now, if I eliminate the LEFT JOIN part of the above, *one* tuple is
returned ... so even with the LEFT JOIN, only *one* tuple is going to be
returned ...

Is there some way to write the above so that it evaluates:

WHERE (n.type = 'A' OR n.type = 'N' OR n.type = 'H' OR n.type = 'C')
AND (nl.id = 15748 AND contact_lvl = 'company')
AND n.nid = nl.nid

first, so that it only has to do the LEFT JOIN on the *one* n.nid that is
returned, instead of the 86736 that are in the table?

Marc G. Fournier ICQ#7615664 IRC Nick: Scrappy
Systems Administrator @ hub.org
primary: scrappy(at)hub(dot)org secondary: scrappy(at){freebsd|postgresql}.org

The Hermit Hacker <scrappy(at)hub(dot)org> writes:
> FROM note_links nl, notes n LEFT JOIN calendar c ON (n.nid = c.nid)
> WHERE (n.type = 'A' OR n.type = 'N' OR n.type = 'H' OR n.type = 'C')
> AND (nl.id = 15748 AND contact_lvl = 'company')
> AND n.nid = nl.nid
> ORDER BY start DESC;

> Is there some way to write the above so that it evaluates:
> first, so that it only has to do the LEFT JOIN on the *one* n.nid that is
> returned, instead of the 86736 that are in the table?

Try adding ... AND n.nid = 15748 ... to the WHERE. It's not very
bright about making that sort of transitive-equality deduction for
itself...

regards, tom lane

I think that using INNER JOIN between nl and n (on n.nid=nl.nid) or
joining those tables in a subquery might work.

On Mon, 18 Jun 2001, The Hermit Hacker wrote:

> Is there some way to write the above so that it evaluates:
>
> WHERE (n.type = 'A' OR n.type = 'N' OR n.type = 'H' OR n.type = 'C')
> AND (nl.id = 15748 AND contact_lvl = 'company')
> AND n.nid = nl.nid
>
> first, so that it only has to do the LEFT JOIN on the *one* n.nid that is
> returned, instead of the 86736 that are in the table?

On Mon, 18 Jun 2001, Tom Lane wrote:

> The Hermit Hacker <scrappy(at)hub(dot)org> writes:
> > FROM note_links nl, notes n LEFT JOIN calendar c ON (n.nid = c.nid)
> > WHERE (n.type = 'A' OR n.type = 'N' OR n.type = 'H' OR n.type = 'C')
> > AND (nl.id = 15748 AND contact_lvl = 'company')
> > AND n.nid = nl.nid
> > ORDER BY start DESC;
>
> > Is there some way to write the above so that it evaluates:
> > first, so that it only has to do the LEFT JOIN on the *one* n.nid that is
> > returned, instead of the 86736 that are in the table?
>
> Try adding ... AND n.nid = 15748 ... to the WHERE. It's not very
> bright about making that sort of transitive-equality deduction for
> itself...

n.nid is the note id ... nl.id is the contact id ...

I'm trying to pull out all notes for the company with an id of 15748:

sepick=# select * from note_links where id = 15748;
nid | id | contact_lvl | owner
-------+-------+-------------+-------
84691 | 15748 | company | f
(1 row)

The Hermit Hacker <scrappy(at)hub(dot)org> writes:
>> Try adding ... AND n.nid = 15748 ... to the WHERE.

> n.nid is the note id ... nl.id is the contact id ...

Ooops, I misread "n.nid = nl.nid" as "n.nid = nl.id". Sorry for the
bogus advice.

Try rephrasing as

FROM (note_links nl JOIN notes n ON (n.nid = nl.nid))
LEFT JOIN calendar c ON (n.nid = c.nid)
WHERE ...

The way you were writing it forced the LEFT JOIN to be done first,
whereas what you want is for the note_links-to-notes join to be done
first. See
http://www.ca.postgresql.org/users-lounge/docs/7.1/postgres/explicit-joins.html

regards, tom lane

Perfect, thank you ... i knew I was overlooking something obvious ... the
query just flies now ...

On Mon, 18 Jun 2001, Tom Lane wrote:

> The Hermit Hacker <scrappy(at)hub(dot)org> writes:
> >> Try adding ... AND n.nid = 15748 ... to the WHERE.
>
> > n.nid is the note id ... nl.id is the contact id ...
>
> Ooops, I misread "n.nid = nl.nid" as "n.nid = nl.id". Sorry for the
> bogus advice.
>
> Try rephrasing as
>
> FROM (note_links nl JOIN notes n ON (n.nid = nl.nid))
> LEFT JOIN calendar c ON (n.nid = c.nid)
> WHERE ...
>
> The way you were writing it forced the LEFT JOIN to be done first,
> whereas what you want is for the note_links-to-notes join to be done
> first. See
> http://www.ca.postgresql.org/users-lounge/docs/7.1/postgres/explicit-joins.html
>
> regards, tom lane
>

Marc G. Fournier ICQ#7615664 IRC Nick: Scrappy
Systems Administrator @ hub.org
primary: scrappy(at)hub(dot)org secondary: scrappy(at){freebsd|postgresql}.org