planner/optimizer question

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 regardless of crm_id 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' and am running postgres
7.4.2.

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