Arnaud Lesauvage <arnaud.lis...@codata.eu> writes:
> First query :

> "Merge Join  (cost=699826.38..704333.80 rows=13548 width=836)"
> "  Merge Cond: (((c.rue)::text = (r.rue)::text) AND ((c.codesite)::text 
> = (r.codesite)::text))"
> "  ->  Sort  (cost=696320.21..697701.07 rows=552343 width=823)"
> "        Sort Key: c.rue, c.codesite"
> "        ->  Seq Scan on cellules c  (cost=0.00..443520.43 rows=552343 
> width=823)"
> "  ->  Sort  (cost=3504.88..3596.96 rows=36833 width=43)"
> "        Sort Key: r.rue, r.codesite"
> "        ->  Seq Scan on rues r  (cost=0.00..711.33 rows=36833 width=43)"

> Second query :

> "Seq Scan on cellules c  (cost=0.00..5018080.39 rows=552343 width=823)"
> "  SubPlan 1"
> "    ->  Index Scan using idx_rues_ruecodesite on rues r 
> (cost=0.00..8.28 rows=1 width=13)"
> "          Index Cond: (((rue)::text = ($1)::text) AND ((codesite)::text 
> = ($0)::text))"

Please notice that the planner thinks the second plan is much more
expensive than the first.  I think you will find that it's right.
Using an index is not always the best way to do a query.

                        regards, tom lane

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