Hi all,

On v7.4.5 I noticed downgrade in the planner, namely favoring
sequential scan over index scan. The proof:

   create table a ( a integer);
   create index aidx on a(a);
   explain analyze select * from a where a = 0;
   -- Index Scan using aidx on a  (cost=0.00..17.07 rows=5 width=4) (actual
   --   time=0.029..0.029 rows=0 loops=1)
   -- Index Cond: (a = 0)
   vacuum analyze;
   explain analyze select * from a where a = 0;
   -- Seq Scan on a (cost=0.00..0.00 rows=1 width=4) (actual time=0.009..0.009 
   --   rows=0 loops=1)
   -- Filter: (a = 0)

I do realize that there might be reasons why this happens over an empty
table, but what is way worse that when the table starts actually to fill,
the seq scan is still there, and the index is simply not used. How
that could be so ...mmm... shortsighted, and what is more important, 
how to avoid this? I hope the answer is not 'run vacuum analyze each 5 seconds'.

        Dmitry Karasik

catpipe Systems ApS
*BSD solutions, consulting, development
+45 7021 0050  

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TIP 9: the planner will ignore your desire to choose an index scan if your
      joining column's datatypes do not match

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