explain EXPLAIN?

explain EXPLAIN?

Post by David Olbers » Sat, 10 Mar 2001 08:23:02

  I'm looking for a better tutorial of how EXPLAIN works. I know Mr. Tom Lane
  wrote a "quick & dirty explanation" and that "plan-reading is an art that
  deserves a tutorial, and I haven't had time to write one". In which case I'd
  like to know if there's any other tutorials/resources.

  I think I get the jist of it (an index scan is better than a seq scan?) but
  I'd like to read more. Does anybody have any suggestions?

-- Dave

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1. Explaining explain ...

hi I'm trying to work out why the following query takes a few seconds to
return results.

this is the query:

            contact_types.description as contact_type,
            people_titles.description as title
            INNER JOIN contact_details ON org_contacts.org_contact_id =
            INNER JOIN contact_types ON contact_details.contact_type =
            LEFT OUTER JOIN people ON contact_details.person_id =
            LEFT OUTER JOIN people_titles ON people.title =
            org_contacts.org_id =  50;

this is what i get from explain

Hash Join  (cost=5384.13..137693.07 rows=3153185 width=92)
  ->  Merge Join  (cost=5383.04..11564.51 rows=3153185 width=76)
        ->  Index Scan using people_pkey on people  (cost=0.00..5206.05
rows=73759 width=32)
        ->  Sort  (cost=5383.04..5383.04 rows=4275 width=44)
              ->  Hash Join  (cost=111.70..5125.22 rows=4275 width=44)
                    ->  Merge Join  (cost=110.61..2681.20 rows=61071
                          ->  Index Scan using
org_contact_id_contact_details_ on contact_details  (cost=0.00..2119.46 r
ows=35920 width=24)
                          ->  Sort  (cost=110.61..110.61 rows=170 width=4)
                                ->  Index Scan using org_id_org_contacts_key
on org_contacts  (cost=0.00..104.32 rows=17
0 width=4)
                    ->  Hash  (cost=1.07..1.07 rows=7 width=16)
                          ->  Seq Scan on contact_types  (cost=0.00..1.07
rows=7 width=16)
  ->  Hash  (cost=1.07..1.07 rows=7 width=16)
        ->  Seq Scan on people_titles  (cost=0.00..1.07 rows=7 width=16)

What i don't understand is the Hash join on the first row (rows 3153185 ?)
and the sort ... can anyone help ?


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