Re: Slow query and using wrong index, how to fix? Probably naive question.. - Mailing list pgsql-general

You might want to try with UNION and then sort the result of this query.

The index history_lookup_lookupid_creator_index wont be used when you are having an "OR" in your WHERE statement.

 select history.id, history.created, creator, contact, history.type, lookup, lookupid, value
from history
where (lookup = 'phone' and lookupid = '672')
union
 select history.id, history.created, creator, contact, history.type, lookup, lookupid, value
from history
where  creator = '790'

Leif Gunnar Erlandsen



________________________________________
Fra: pgsql-general-owner@postgresql.org [pgsql-general-owner@postgresql.org] på vegne av Antonio Goméz Soto
[antonio.gomez.soto@gmail.com]
Sendt: 22. mai 2013 10:50
Til: pgsql-general@postgresql.org
Emne: [GENERAL] Slow query and using wrong index, how to fix? Probably naive question..

Hi,

I am using postgresql 8.1 (CentOS5). I have the following table:

system # \d history
                                   Table "public.history"
  Column  |           Type           |                      Modifiers
----------+--------------------------+------------------------------------------------------
 id       | integer                  | not null default nextval('history_id_seq'::regclass)
 created  | timestamp with time zone |
 creator  | integer                  | not null default 1
 contact  | integer                  | not null default 1
 type     | character varying        | not null default ''::character varying
 lookup   | text                     |
 lookupid | integer                  | not null default 1
 value    | text                     |
Indexes:
    "history_pkey" PRIMARY KEY, btree (id)
    "history_created_index" btree (created)
    "history_creator_index" btree (creator)
    "history_lookup_lookupid_creator_index" btree (lookup, lookupid, creator)
    "history_lookup_lookupid_index" btree (lookup, lookupid)
Foreign-key constraints:
    "history_contact_constraint" FOREIGN KEY (contact) REFERENCES contact(id)
    "history_creator_constraint" FOREIGN KEY (creator) REFERENCES contact(id)

system # explain select history.id, history.created, creator, contact, history.type, lookup, lookupid, value from
history where (lookup = 'phone' and lookupid = '672') or creator = '790' order by history.creator desc limit 1000; 
                                                   QUERY PLAN
----------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.00..132041.59 rows=1000 width=58)
   ->  Index Scan Backward using history_creator_index on history  (cost=0.00..11746815.97 rows=88963 width=58)
         Filter: (((lookup = 'phone'::text) AND (lookupid = 672)) OR (creator = 790))
(3 rows)

This table contains 2 million rows, the query takes 800 seconds on SSD HD.

I think - probably naive - the query should use the history_lookup_lookupid_creator_index.

Why doesn't it, and how can I speed up the query?

Thanks,
Antonio.




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