[SQL] how can I improve the speed of this query

[Peter T. Brown]

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)
                          ->  Nested Loop  (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=19 width=8)
              ->  Index Scan using VisitorInternetDeviceAssoc_Visi on
                  VisitorInternetDeviceAssoc  (cost=0.00..16402.90 rows=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)



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