Index Scans become Seq Scans after VACUUM ANALYSE - Mailing list pgsql-hackers

Hello,

While trying to optimise a query I found that running VACUUM ANALYSE
changed all the Index Scans to Seq Scans and that the only way to revert
to Index Scans was the add "enable_seqscan = 0" in postgresql.conf.

Seq Scans are much slower for that specific query. Why does Postgres
switch to that method?
PostgreSQL 7.2.1 on i686-pc-linux-gnu, compiled by GCC 2.95.4
(1 row)

Output with "enable_seqscan = 0":
gesci5=# explain select p.id_prospect, p.position_prospect, initcap(p1.nom) as nom, initcap(p1.prenom) as prenom,
a1.no_tel,a1.no_portable, p.dernier_contact, cn.id_contact, cn.id_vendeur, cn.id_operation, case when p.dernier_contact
isnot null then cn.date_contact::abstime::int4 else p.cree_le::abstime::int4 end as date_contact, cn.type_contact,
cn.nouveau_rdv::abstime::int4as nouveau_rdv, cn.date_echeance::abstime::int4 as date_echeance,
cn.date_reponse::abstime::int4as date_reponse from prospect p left join personne p1 on (p1.id_personne =
p.id_personne1)left join adresse a1 on (a1.id_adresse = p1.id_adresse_principale) left join contact cn on
(p.dernier_contact= cn.id_contact) where (p.abandon is null or p.abandon != 'O')  order by cn.date_contact desc;NOTICE:
QUERY PLAN:
 
Sort  (cost=49442.99..49442.99 rows=24719 width=123)  ->  Hash Join  (cost=14146.79..46656.05 rows=24719 width=123)
  ->  Merge Join  (cost=9761.33..40724.83 rows=24719 width=66)              ->  Sort  (cost=9761.33..9761.33 rows=24719
width=49)                   ->  Merge Join  (cost=0.00..7485.53 rows=24719 width=49)                          ->  Index
Scanusing prospect_personne1 on prospect p  (cost=0.00..4322.18 rows=24719 width=22)                          ->  Index
Scanusing personne_pkey on personne p1  (cost=0.00..2681.90 rows=44271 width=27)              ->  Index Scan using
adresse_pkeyon adresse a1  (cost=0.00..30354.16 rows=95425 width=17)        ->  Hash  (cost=3242.09..3242.09 rows=30224
width=57)             ->  Index Scan using contact_pkey on contact cn  (cost=0.00..3242.09 rows=30224 width=57)
 

Output with "enable_seqscan = 1": 
Sort  (cost=18622.67..18622.67 rows=24719 width=123)  ->  Hash Join  (cost=10034.30..15835.73 rows=24719 width=123)
  ->  Hash Join  (cost=8074.99..12330.65 rows=24719 width=66)              ->  Hash Join  (cost=2088.54..4629.65
rows=24719width=49)                    ->  Seq Scan on prospect p  (cost=0.00..1289.35 rows=24719 width=22)
      ->  Hash  (cost=1106.71..1106.71 rows=44271 width=27)                          ->  Seq Scan on personne p1
(cost=0.00..1106.71rows=44271 width=27)              ->  Hash  (cost=2561.25..2561.25 rows=95425 width=17)
     ->  Seq Scan on adresse a1  (cost=0.00..2561.25 rows=95425 width=17)        ->  Hash  (cost=1036.24..1036.24
rows=30224width=57)              ->  Seq Scan on contact cn  (cost=0.00..1036.24 rows=30224 width=57)
 



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