index usage - Mailing list pgsql-performance

Hi,

I have a query which I think should be using an index all of the time but
postgres only uses the index part of the time.  The index
(ticket_crm_map_crm_id_suppid) has the where clause column (crm_id) listed
first followed by the selected column (support_person_id).  Wouldn't the
most efficient plan be to scan the index each time because the only columns
needed are in the index?  Below is the table, 2 queries showing the
difference in plans, followed by the record distribution of ticket_crm_map.
I first did a 'vacuum analyze' to update the statistics.

Thanks,
Brad


athenapost=> \d ticket_crm_map
                                  Table "public.ticket_crm_map"
        Column         |            Type             |
Modifiers
 ------------------------+-----------------------------+---------------------
 -----------------------
tcrm_map_id            | integer                     | not null
ticket_id              | integer                     | not null
crm_id                 | integer                     | not null
support_person_id      | integer                     | not null
escalated_to_person_id | integer                     | not null
status                 | character varying(50)       | not null default
'Open'::character varying
close_date             | timestamp without time zone |
updated_date           | timestamp without time zone |
updated_by             | character varying(255)      |
created_date           | timestamp without time zone |
created_by             | character varying(255)      |
additional_info        | text                        |
subject                | character varying(255)      |
Indexes:
   "ticket_crm_map_pkey" primary key, btree (tcrm_map_id)
   "ticket_crm_map_crm_id_key" unique, btree (crm_id, ticket_id)
   "ticket_crm_map_crm_id_suppid" btree (crm_id, support_person_id)
   "ticket_crm_map_status" btree (status)
   "ticket_crm_map_ticket_id" btree (ticket_id)
Foreign-key constraints:
   "$1" FOREIGN KEY (ticket_id) REFERENCES ticket(ticket_id)
   "$2" FOREIGN KEY (crm_id) REFERENCES company_crm(crm_id)
   "$3" FOREIGN KEY (support_person_id) REFERENCES person(person_id)
   "$4" FOREIGN KEY (escalated_to_person_id) REFERENCES person(person_id)
   "$5" FOREIGN KEY (status) REFERENCES ticket_status(status)

athenapost=> explain analyze select distinct support_person_id from
ticket_crm_map where crm_id = 7;

QUERY PLAN
 ----------------------------------------------------------------------------
 ----------------------------------------------------------------------------
 ----------
Unique  (cost=1262.99..1265.27 rows=1 width=4) (actual time=15.335..18.245
rows=20 loops=1)
  ->  Sort  (cost=1262.99..1264.13 rows=456 width=4) (actual
time=15.332..16.605 rows=2275 loops=1)
        Sort Key: support_person_id
        ->  Index Scan using ticket_crm_map_crm_id_suppid on ticket_crm_map
(cost=0.00..1242.85 rows=456 width=4) (actual time=0.055..11.281 rows=2275
loops=1)
              Index Cond: (crm_id = 7)
Total runtime: 18.553 ms
(6 rows)

Time: 20.598 ms
athenapost=> explain analyze select distinct support_person_id from
ticket_crm_map where crm_id = 1;
                                                          QUERY PLAN
 ----------------------------------------------------------------------------
 -----------------------------------------------------
Unique  (cost=10911.12..11349.26 rows=32 width=4) (actual
time=659.102..791.517 rows=24 loops=1)
  ->  Sort  (cost=10911.12..11130.19 rows=87628 width=4) (actual
time=659.090..713.285 rows=93889 loops=1)
        Sort Key: support_person_id
        ->  Seq Scan on ticket_crm_map  (cost=0.00..3717.25 rows=87628
width=4) (actual time=0.027..359.299 rows=93889 loops=1)
              Filter: (crm_id = 1)
Total runtime: 814.601 ms
(6 rows)

Time: 817.095 ms
athenapost=> select count(*), crm_id from ticket_crm_map group by crm_id;
count | crm_id
 -------+--------
 2554 |     63
  129 |     25
   17 |     24
  110 |     23
   74 |     22
   69 |     21
    2 |     20
   53 |     82
   10 |     17
   16 |     81
46637 |     16
   14 |     80
    2 |     15
 1062 |     79
   87 |     78
   93 |     77
   60 |     44
  363 |     76
  225 |     10
    4 |     74
   83 |      9
   27 |     73
  182 |      8
 2275 |      7
   15 |     71
  554 |      6
   44 |     70
  631 |      5
   37 |      4
  190 |      3
  112 |      2
93889 |      1
(32 rows)

Time: 436.697 ms