Question about index usage

[Guido Neitzer <guido.neitzer@xxxxxxxxxxxxx>]

Hi.

Is there a reason why this query:

select id from dga_dienstleister where plz in ('45257', '45259');

doesn't use this index:

    "dga_dienstleister_plz_index" btree (plz varchar_pattern_ops)

but uses this index:

    "dga_dienstleister_plz_index2" btree (plz)

I had the first index setup for queries with "plz like '4525%'" but I  
never tested the "in" query until I saw in the logs that these  
queries where slow compared to the rest. Query plans at the end.

cug


DGADB=# explain analyse select id from dga_dienstleister where plz  
like  
'45257';                                                               Q 
UERY PLAN
------------------------------------------------------------------------ 
----------------------------------------------------------------
Bitmap Heap Scan on dga_dienstleister  (cost=2.07..82.41 rows=21  
width=8) (actual time=13.489..14.211 rows=16 loops=1)
   Filter: ((plz)::text ~~ '45257'::text)
   ->  Bitmap Index Scan on dga_dienstleister_plz_index   
(cost=0.00..2.07 rows=21 width=0) (actual time=13.323..13.323 rows=16  
loops=1)
         Index Cond: ((plz)::text ~=~ '45257'::character varying)
Total runtime: 14.328 ms
(5 rows)


DGADB=# explain analyse select id from dga_dienstleister where plz =  
'45257';
                                                              QUERY  
PLAN
------------------------------------------------------------------------ 
---------------------------------------------------------------
Bitmap Heap Scan on dga_dienstleister  (cost=2.07..82.41 rows=21  
width=8) (actual time=0.486..0.663 rows=16 loops=1)
   Recheck Cond: ((plz)::text = '45257'::text)
   ->  Bitmap Index Scan on dga_dienstleister_plz_index2   
(cost=0.00..2.07 rows=21 width=0) (actual time=0.424..0.424 rows=16  
loops=1)
         Index Cond: ((plz)::text = '45257'::text)
Total runtime: 0.826 ms
(5 rows)

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