Very slow query

The following query on some small datasets takes over a second to run.
I'd be grateful for some help in understanding the explain output, and
to remake the code.

Looks like the sort is using up quite a bit of resources. I made an
index on boards using columns "b.n_type, b.n_id, b.t_name" but the index
was not recorded in explain analyze. (see "testindex" below).

I am using PostgreSQL 7.4.2 on i386-pc-linux-gnu (Debian). The query is
in a psql function (I've changed variables to hard coded integers
below). The db is vacuumed every night.

Thanks for any help;
Rory


SELECT DISTINCT
        b.n_id                           as id,
        b.n_type,
        CASE b.n_type WHEN 0 THEN 'personal'
                      WHEN 1 THEN 'private'
                      WHEN 2 THEN 'blog'
                      ELSE 'public'
                      END                as type,
        b.t_name                         as title
    FROM
        boards b, people p, objects o
    WHERE
         b.b_hidden = 'f'
         AND
         (
            b.n_type = 3
            OR
            b.n_creator = 71
            OR
            (   b.n_id = o.n_board_id
                AND
                o.n_creator = 71
                AND
                o.n_joined > 0
            )
         )
    ORDER BY
        b.n_type, b.n_id ASC, b.t_name;

trial=> \d boards
                                         Table "public.boards"
    Column     |            Type             |                        Modifiers
---------------+-----------------------------+----------------------------------------------------------
 n_id          | integer                     | not null default nextval('public.boards_n_id_seq'::text)
 b_hidden      | boolean                     | default false
 dt_created    | timestamp without time zone | default ('now'::text)::timestamp(6) with time zone
 dt_modified   | timestamp without time zone | default ('now'::text)::timestamp(6) with time zone
 t_mobile      | character varying(15)       |
 t_email       | character varying(50)       |
 n_creator     | integer                     | not null
 n_type        | smallint                    | not null default 0
 t_name        | character varying(100)      | not null
 t_description | character varying(500)      |
 n_id_photo    | integer                     |
 n_bg_colour   | integer                     | default 0
Indexes:
    "boards_pkey" primary key, btree (n_id)
    "boards_t_email_key" unique, btree (t_email)
    "boards_t_mobile_key" unique, btree (t_mobile)
    "testindex" btree (n_type, n_id, t_name)
Foreign-key constraints:
    "$1" FOREIGN KEY (n_creator) REFERENCES people(n_id) ON UPDATE CASCADE ON DELETE CASCADE
    "$2" FOREIGN KEY (n_id_photo) REFERENCES photo(n_id) ON UPDATE CASCADE ON DELETE SET NULL
Triggers:
    tr_update_modified_time BEFORE UPDATE ON boards FOR EACH ROW EXECUTE PROCEDURE fn_update_modified_time()

trial=> \d people
                                           Table "public.people"
      Column       |            Type             |                        Modifiers
-------------------+-----------------------------+----------------------------------------------------------
 n_id              | integer                     | not null default nextval('public.people_n_id_seq'::text)
 n_object_id       | integer                     |
 n_objects_counter | integer                     | default 0
 b_hidden          | boolean                     | default false
 dt_created        | timestamp without time zone | default ('now'::text)::timestamp(6) with time zone
 dt_modified       | timestamp without time zone | default ('now'::text)::timestamp(6) with time zone
 t_nickname        | character varying(20)       | not null
 t_firstname       | character varying(20)       | not null
 t_surname         | character varying(25)       | not null
 t_mobile          | character varying(15)       |
 t_email           | character varying(50)       |
 t_password        | character varying(15)       | not null
 b_registered      | boolean                     | default false
 n_email_status    | smallint                    | default 0
 n_txt_status      | smallint                    | default 0
 b_work_hours      | boolean                     | default false
 b_confirmations   | boolean                     | default true
Indexes:
    "people_pkey" primary key, btree (n_id)
    "people_t_email_key" unique, btree (t_email)
    "people_t_mobile_key" unique, btree (t_mobile)
    "people_t_nickname_key" unique, btree (t_nickname)
Foreign-key constraints:
    "object_chk" FOREIGN KEY (n_object_id) REFERENCES objects(n_id) ON UPDATE CASCADE ON DELETE SET NULL
Triggers:
    tr_update_modified_time BEFORE UPDATE ON people FOR EACH ROW EXECUTE PROCEDURE fn_update_modified_time()

trial=> \d objects
                                         Table "public.objects"
    Column     |            Type             |                         Modifiers
---------------+-----------------------------+-----------------------------------------------------------
 n_id          | integer                     | not null default nextval('public.objects_n_id_seq'::text)
 t_text_id     | character varying(25)       | not null
 b_hidden      | boolean                     | default false
 dt_created    | timestamp without time zone | default ('now'::text)::timestamp(6) with time zone
 dt_modified   | timestamp without time zone | default ('now'::text)::timestamp(6) with time zone
 n_creator     | integer                     | not null
 n_type        | smallint                    | not null default 0
 t_name        | character varying(100)      | not null
 t_description | text                        |
 t_postcode    | character varying(10)       |
 n_id_photo    | integer                     |
 n_board_id    | integer                     | not null
 n_joined      | smallint                    |
 b_trigger     | boolean                     | default true
Indexes:
    "objects_pkey" primary key, btree (n_id)
Foreign-key constraints:
    "$1" FOREIGN KEY (n_creator) REFERENCES people(n_id) ON UPDATE CASCADE ON DELETE CASCADE
    "$2" FOREIGN KEY (n_id_photo) REFERENCES photo(n_id) ON UPDATE CASCADE ON DELETE SET NULL
    "$3" FOREIGN KEY (n_board_id) REFERENCES boards(n_id) ON UPDATE CASCADE ON DELETE CASCADE



 Unique  (cost=3677.26..3711.48 rows=102 width=18) (actual time=12566.422..13045.404 rows=4 loops=1)
   ->  Sort  (cost=3677.26..3684.10 rows=2738 width=18) (actual time=12566.413..12809.928 rows=158254 loops=1)
         Sort Key: b.n_type, b.n_id, b.t_name, CASE WHEN (b.n_type = 0) THEN 'personal'::text WHEN (b.n_type = 1) THEN
'private'::te
xt WHEN (b.n_type = 2) THEN 'blog'::text ELSE 'public'::text END
         ->  Nested Loop  (cost=3442.79..3520.93 rows=2738 width=18) (actual time=0.244..1052.180 rows=158254 loops=1)
               ->  Seq Scan on people p  (cost=0.00..2.67 rows=67 width=0) (actual time=0.012..0.709 rows=67 loops=1)
               ->  Materialize  (cost=3442.79..3443.20 rows=41 width=18) (actual time=0.005..6.528 rows=2362 loops=67)
                     ->  Nested Loop  (cost=3.27..3442.79 rows=41 width=18) (actual time=0.216..273.709 rows=2362
loops=1)
                           Join Filter: ((("inner".n_id = "outer".n_board_id) OR ("inner".n_type = 3) OR
("inner".n_creator= 71)) A 
ND (("outer".n_creator = 71) OR ("inner".n_type = 3) OR ("inner".n_creator = 71)) AND (("outer".n_joined > 0) OR
("inner".n_type= 3 
) OR ("inner".n_creator = 71)))
                           ->  Seq Scan on objects o  (cost=0.00..27.87 rows=787 width=10) (actual time=0.005..1.372
rows=787loops= 
1)
                           ->  Materialize  (cost=3.27..4.29 rows=102 width=22) (actual time=0.001..0.105 rows=102
loops=787)
                                 ->  Seq Scan on boards b  (cost=0.00..3.27 rows=102 width=22) (actual
time=0.017..0.352rows=102 lo 
ops=1)
                                       Filter: (b_hidden = false)
 Total runtime: 13051.065 ms



--
Rory Campbell-Lange
<rory@campbell-lange.net>
<www.campbell-lange.net>