how can I improve the speed of this query - Mailing list pgsql-sql

Hi--

I have this rather long complex query that takes really long to complete
(please see below). It seems like I ought to improve the speed somehow.
I don't understand, for example, what the query planner is doing when it
says "Hash" and why this appears to take so long. And since I have a key
for Visitor.ID, I don't understand why its doing a sequential scan on
that table... 

Any advice would be greatly appreciated!


Thanks

Peter




EXPLAIN SELECT
"Visitor"."Created","Visitor"."Updated","Tidbit"."ID","ProgramEvent"."ID","Visitor"."Email","Interest"."ID","VisitorInternetDeviceAssoc"."ID","Referral"."ID"


FROM "VisitorExtra" 

LEFT OUTER JOIN Tidbit" ON "VisitorExtra"."ID"="Tidbit"."VisitorID" 

LEFT OUTER JOIN "ProgramEvent" ON"VisitorExtra"."ID"="ProgramEvent"."VisitorID" 

LEFT OUTER JOIN "Interest" ON "VisitorExtra"."ID"="Interest"."VisitorID" 

LEFT OUTER JOIN "VisitorInternetDeviceAssoc" ON"VisitorExtra"."ID"="VisitorInternetDeviceAssoc"."VisitorID" 

LEFT OUTER JOIN "Referral" ON"VisitorExtra"."ID"="Referral"."FromVisitorID","Visitor" 

WHERE "VisitorExtra"."ID"="Visitor"."ID" AND "VisitorExtra"."ID"= 325903;


NOTICE:  QUERY PLAN:

Hash Join  (cost=14584.37..59037.79 rows=57747 width=76) ->  Merge Join  (cost=0.00..36732.65 rows=57747 width=44)
->  Merge Join  (cost=0.00..29178.16 rows=10681 width=36)             ->  Nested Loop  (cost=0.00..10505.74 rows=6674
width=28)                  ->  Nested Loop  (cost=0.00..435.29 rows=177        width=20)                         ->
NestedLoop  (cost=0.00..15.70 rows=55            width=12)                               ->  Index Scan using
VisitorExtra_pkey               on VisitorExtra  (cost=0.00..3.01 rows=1                     width=4)
           ->  Index Scan using                    
 
Tidbit_VisitorID_key on Tidbit                      (cost=0.00..12.67 rows=2
width=8)                         ->  Index Scan using                                        
ProgramEvent_VisitorID_key on ProgramEvent                    (cost=0.00..7.57
rows=2 width=8)                   ->  Index Scan using Interest_VisitorID_key on        Interest  (cost=0.00..56.66
rows=19width=8)             ->  Index Scan using VisitorInternetDeviceAssoc_Visi on      VisitorInternetDeviceAssoc
(cost=0.00..16402.90rows=174887          
 
width=8)       ->  Index Scan using Referral_FromVisitorID_key on Referral     (cost=0.00..6323.41 rows=87806 width=8)
-> Hash  (cost=6061.79..6061.79 rows=317379 width=32)       ->  Seq Scan on Visitor  (cost=0.00..6061.79 rows=317379
   
 
width=32)