Обсуждение: 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
ISNULL THEN date_part('epoch', n.added) ELSE
date_part('epoch',c.act_start) END AS start FROM note_links nl,
notesn 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')
ANDn.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
Scanusing note_links_id on note_links nl (cost=0.00..3.49 rows=1 width=16) -> Materialize
(cost=6692.63..6692.63rows=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..206.22rows=10122 width=36) -> Seq Scan on calendar c (cost=0.00..206.22
rows=10122width=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@hub.org secondary: scrappy@{freebsd|postgresql}.org
The Hermit Hacker <scrappy@hub.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@hub.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@hub.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@hub.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@hub.org secondary: scrappy@{freebsd|postgresql}.org