Optimizer Not using the Right plan

Hi,

     Postgres 8.2.4 is not using the right plan for different values.

    From the below queries listing.addressvaluation table has 19million
records , the other table listing.valuationchangeperiod is just lookup
table with 3 records.

    If you can see the explain plans for the statements the first one
uses a bad plan for  737987 addressid search, does a index scan backward
on the primary key "addressvaluationid" takes more time to execute and
the same query for a different addressid (5851202) uses the correct
optimal plan with index scan on "addressid" column which is way quicker.

   Autovacuums usually vacuums these tables regularly, in fact I checked
the pg_stat_user_tables the last vacuum/analyze on this table was last
night.
   I did another manual vacuum analyze on the listing.addrevaluation
table it uses the right plan for all the values now.

   Can anyone explain me this wierd behavior ?
   why does it have different plans for different values and after doing
manual vacuum analyze it works properly ?

    Are autovacuums not effective enough ?

   Here are my autovacuum settings

autovacuum_naptime = 120min
autovacuum_vacuum_threshold = 500
autovacuum_analyze_threshold = 250
autovacuum_vacuum_scale_factor = 0.001
autovacuum_analyze_scale_factor = 0.001
autovacuum_freeze_max_age = 200000000
autovacuum_vacuum_cost_delay = -1
autovacuum_vacuum_cost_limit = -1

   Here are the table structures


 listing.addressvaluation
                                                   Table
"listing.addressvaluation"
           Column           |            Type
|                                 Modifiers

----------------------------+-----------------------------+---------------------------------------------------------------------------
 addressvaluationid         | integer                     | not null
default nextval(('listing.addressvaluationseq'::text)::regclass)
 fkaddressid                | integer                     | not null
 fkaddressvaluationsourceid | integer                     | not null
 sourcereference            | text                        |
 createdate                 | timestamp without time zone | not null
default ('now'::text)::timestamp(6) without time zone
 valuationdate              | timestamp without time zone | not null
 valuationamount            | numeric(14,2)               |
 valuationhigh              | numeric(14,2)               |
 valuationlow               | numeric(14,2)               |
 valuationconfidence        | integer                     |
 valuationchange            | numeric(14,2)               |
 fkvaluationchangeperiodid  | integer                     |
 historycharturl            | text                        |
 regionhistorycharturl      | text                        |
Indexes:
    "pk_addressvaluation_addressvaluationid" PRIMARY KEY, btree
(addressvaluationid), tablespace "indexdata"
    "idx_addressvaluation_createdate" btree (createdate), tablespace
"indexdata"
    "idx_addressvaluation_fkaddressid" btree (fkaddressid), tablespace
"indexdata"
    "idx_addressvaluation_fkaddressid2" btree (fkaddressid), tablespace
"indexdata"
Foreign-key constraints:
    "fk_addressvaluation_address" FOREIGN KEY (fkaddressid) REFERENCES
listing.address(addressid)
    "fk_addressvaluation_addressvaluationsource" FOREIGN KEY
(fkaddressvaluationsourceid) REFERENCES
listing.addressvaluationsource(addressvaluationsourceid)
    "fk_addressvaluation_valuationchangeperiod" FOREIGN KEY
(fkvaluationchangeperiodid) REFERENCES
listing.valuationchangeperiod(valuationchangeperiodid)

listing.valuationchangeperiod
                                       Table "listing.valuationchangeperiod"
         Column          |  Type   |
Modifiers
-------------------------+---------+--------------------------------------------------------------------------------
 valuationchangeperiodid | integer | not null default
nextval(('listing.valuationchangeperiodseq'::text)::regclass)
 name                    | text    | not null
Indexes:
    "pk_valuationchangeperiod_valuationchangeperiodid" PRIMARY KEY,
btree (valuationchangeperiodid), tablespace "indexdata"
    "uq_valuationchangeperiod_name" UNIQUE, btree (name), tablespace
"indexdata"



For Addressid 737987 after autovacuum before manual vacuum analyze
-------------------------------------------------------------------------------------------
explain
select this_.addressvaluationid as addressv1_150_1_,
this_.sourcereference as sourcere2_150_1_,
          this_.createdate as createdate150_1_, this_.valuationdate as
valuatio4_150_1_,
          this_.valuationamount as valuatio5_150_1_, this_.valuationhigh
as valuatio6_150_1_,
          this_.valuationlow as valuatio7_150_1_,
this_.valuationconfidence as valuatio8_150_1_,
          this_.valuationchange as valuatio9_150_1_,
this_.historycharturl as history10_150_1_,
          this_.regionhistorycharturl as regionh11_150_1_,
this_.fkaddressid as fkaddre12_150_1_,
          this_.fkaddressvaluationsourceid as fkaddre13_150_1_,
this_.fkvaluationchangeperiodid as fkvalua14_150_1_,
         valuationc2_.valuationchangeperiodid as valuatio1_197_0_,
valuationc2_.name as name197_0_
from listing.addressvaluation this_ left outer join
listing.valuationchangeperiod valuationc2_
        on
this_.fkvaluationchangeperiodid=valuationc2_.valuationchangeperiodid
where this_.fkaddressid=737987
order by this_.addressvaluationid
desc limit 1;

QUERY PLAN

--------------------------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.00..678.21 rows=1 width=494)
   ->  Nested Loop Left Join  (cost=0.00..883026.09 rows=1302 width=494)
         ->  Index Scan Backward using
pk_addressvaluation_addressvaluationid on addressvaluation this_
(cost=0.00..882649.43 rows=1302 width=482)
               Filter: (fkaddressid = 737987)
         ->  Index Scan using
pk_valuationchangeperiod_valuationchangeperiodid on
valuationchangeperiod valuationc2_  (cost=0.00..0.28 rows=1 width=12)
               Index Cond: (this_.fkvaluationchangeperiodid =
valuationc2_.valuationchangeperiodid)
(6 rows)


For Addressid 5851202 after autovacuum before manual vacuum analyze
--------------------------------------------------------------------------------------------

select this_.addressvaluationid as addressv1_150_1_,
this_.sourcereference as sourcere2_150_1_,
          this_.createdate as createdate150_1_, this_.valuationdate as
valuatio4_150_1_,
          this_.valuationamount as valuatio5_150_1_, this_.valuationhigh
as valuatio6_150_1_,
          this_.valuationlow as valuatio7_150_1_,
this_.valuationconfidence as valuatio8_150_1_,
          this_.valuationchange as valuatio9_150_1_,
this_.historycharturl as history10_150_1_,
          this_.regionhistorycharturl as regionh11_150_1_,
this_.fkaddressid as fkaddre12_150_1_,
          this_.fkaddressvaluationsourceid as fkaddre13_150_1_,
this_.fkvaluationchangeperiodid as fkvalua14_150_1_,
          valuationc2_.valuationchangeperiodid as valuatio1_197_0_,
valuationc2_.name as name197_0_
from listing.addressvaluation this_ left outer join
listing.valuationchangeperiod valuationc2_
on this_.fkvaluationchangeperiodid=valuationc2_.valuationchangeperiodid
where this_.fkaddressid=5851202
order by this_.addressvaluationid
desc limit 1;
                                                              QUERY PLAN

--------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=30.68..30.68 rows=1 width=494)
   ->  Sort  (cost=30.68..30.71 rows=11 width=494)
         Sort Key: this_.addressvaluationid
         ->  Hash Left Join  (cost=1.07..30.49 rows=11 width=494)
               Hash Cond: (this_.fkvaluationchangeperiodid =
valuationc2_.valuationchangeperiodid)
               ->  Index Scan using idx_addressvaluation_fkaddressid2 on
addressvaluation this_  (cost=0.00..29.27 rows=11 width=482)
                     Index Cond: (fkaddressid = 5851202)
               ->  Hash  (cost=1.03..1.03 rows=3 width=12)
                     ->  Seq Scan on valuationchangeperiod valuationc2_
(cost=0.00..1.03 rows=3 width=12)
(9 rows)


After manual vacuum analyze  for addressid 737987
------------------------------------------------------------------

explain
select this_.addressvaluationid as addressv1_150_1_,
this_.sourcereference as sourcere2_150_1_,
          this_.createdate as createdate150_1_, this_.valuationdate as
valuatio4_150_1_,
          this_.valuationamount as valuatio5_150_1_, this_.valuationhigh
as valuatio6_150_1_,
          this_.valuationlow as valuatio7_150_1_,
this_.valuationconfidence as valuatio8_150_1_,
          this_.valuationchange as valuatio9_150_1_,
this_.historycharturl as history10_150_1_,
          this_.regionhistorycharturl as regionh11_150_1_,
this_.fkaddressid as fkaddre12_150_1_,
          this_.fkaddressvaluationsourceid as fkaddre13_150_1_,
this_.fkvaluationchangeperiodid as fkvalua14_150_1_,
          valuationc2_.valuationchangeperiodid as valuatio1_197_0_,
valuationc2_.name as name197_0_
from listing.addressvaluation this_ inner join
listing.valuationchangeperiod valuationc2_
on this_.fkvaluationchangeperiodid=valuationc2_.valuationchangeperiodid
where this_.fkaddressid=737987
order by this_.addressvaluationid
desc limit 1;
                                                             QUERY PLAN

-------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=31.24..31.24 rows=1 width=494)
   ->  Sort  (cost=31.24..31.27 rows=11 width=494)
         Sort Key: this_.addressvaluationid
         ->  Hash Join  (cost=1.07..31.05 rows=11 width=494)
               Hash Cond: (this_.fkvaluationchangeperiodid =
valuationc2_.valuationchangeperiodid)
               ->  Index Scan using idx_addressvaluation_fkaddressid on
addressvaluation this_  (cost=0.00..29.83 rows=11 width=482)
                     Index Cond: (fkaddressid = 737987)
               ->  Hash  (cost=1.03..1.03 rows=3 width=12)
                     ->  Seq Scan on valuationchangeperiod valuationc2_
(cost=0.00..1.03 rows=3 width=12)
(9 rows)



Thanks!
Pallav.