Thread: Very simple select, using index for ordering, but not for selecting. How to make it faster?

Hi,

I have the following simple query on a simple table:

system=# select * from history where lookup = 'trunk' and lookupid = '248' order by created desc limit 1000;


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 * from history where lookup = 'trunk' and lookupid = '248' order by created desc limit 1000;
                                              QUERY PLAN 
-----------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.00..14799.28 rows=1000 width=58)
   ->  Index Scan Backward using history_created_index on history  (cost=0.00..12201987.90 rows=824499 width=58)
         Filter: ((lookup = 'trunk'::text) AND (lookupid = 248))
(3 rows)


Why doesn't it use the index specified in select? How can I change the query to make it faster?

Thanks,
Antonio

On 05/22/2013 02:38 PM, Antonio Goméz Soto wrote:

> Limit  (cost=0.00..14799.28 rows=1000 width=58) ->  Index Scan
> Backward using history_created_index on history
> (cost=0.00..12201987.90 rows=824499 width=58) Filter: ((lookup =
> 'trunk'::text) AND (lookupid = 248))

It's not using history_lookup_lookupid_creator_index, or even
history_lookup_lookupid_index, because it thinks, rightly or wrongly,
that it can get 1000 rows by reading history_creator_index backwards and
filtering out rows that don't match your where clause.

Since in this case, ordering is the most beneficial piece, it can't use
history_lookup_lookupid_creator_index to do this because creator is the
third column in the index. If you redefine that index to this instead:

CREATE INDEX history_lookup_lookupid_creator_index
     ON public.history (creator, lookup, lookupid);

You *should* get a much faster result. That would also allow you to drop
history_creator_index. Since history_lookup_lookupid_index covers the
same first two columns, you shouldn't lose anything in queries that work
better with those in the front.

--
Shaun Thomas
OptionsHouse | 141 W. Jackson Blvd. | Suite 500 | Chicago IL, 60604
312-676-8870
sthomas@optionshouse.com

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