Doing some testing on upcoming 8.1 devel and am having serious issues
with new bitmap index scan feature.  It is easy to work around (just
disable it) but IMO the planner is using it when a regular index scan
should be strongly favored.  The performance of the bitmapscan in my
usage is actually quite a bit worse than a full sequential scan.

here is a query which does this:
explain analyze execute
data1_read_next_product_structure_file_0('012241', '', '', '002', 1);

Here is the 8.0/bitmap off plan:
Limit  (cost=0.00..45805.23 rows=5722 width=288) (actual
time=0.070..0.072 rows=1 loops=1)
   ->  Index Scan using product_structure_file_pkey on
product_structure_file  (cost=0.00..45805.23 rows=5722 width=288)
(actual time=0.063..0.063 row
s=1 loops=1)
         Index Cond: ((ps_parent_code)::text >= ($1)::text)
         Filter: ((((ps_parent_code)::text > ($1)::text) OR
(ps_group_code >= $2)) AND (((ps_parent_code)::text > ($1)::text) OR
(ps_group_code > $2)
OR ((ps_section_code)::text >= ($3)::text)) AND (((ps_parent_code)::text
> ($1)::text) OR (ps_group_code > $2) OR ((ps_section_code)::text >
t) OR ((ps_seq_no)::smallint > $4)))
 Total runtime: 0.185 ms

Here is the 8.1 with bitamp on:
Limit  (cost=3768.32..3782.63 rows=5722 width=288) (actual
time=2287.488..2287.490 rows=1 loops=1)
   ->  Sort  (cost=3768.32..3782.63 rows=5722 width=288) (actual
time=2287.480..2287.480 rows=1 loops=1)
         Sort Key: ps_parent_code, ps_group_code, ps_section_code,
         ->  Bitmap Heap Scan on product_structure_file
(cost=187.84..3411.20 rows=5722 width=288) (actual time=19.977..514.532
rows=47355 loops=1)
               Recheck Cond: ((ps_parent_code)::text >= ($1)::text)
               Filter: ((((ps_parent_code)::text > ($1)::text) OR
(ps_group_code >= $2)) AND (((ps_parent_code)::text > ($1)::text) OR
> $2) OR ((ps_section_code)::text >= ($3)::text)) AND
(((ps_parent_code)::text > ($1)::text) OR (ps_group_code > $2) OR
((ps_section_code)::text > ($3
)::text) OR ((ps_seq_no)::smallint > $4)))
               ->  Bitmap Index Scan on product_structure_file_pkey
(cost=0.00..187.84 rows=18239 width=0) (actual time=19.059..19.059
rows=47356 loo
                     Index Cond: ((ps_parent_code)::text >= ($1)::text)
 Total runtime: 2664.034 ms

Here is the prepared statement definition:
prepare data1_read_next_product_structure_file_0 (character varying,
character, character varying, int4, int4)
        as select 1::int4, * from data1.product_structure_file
        where ps_parent_code >= $1 and 
                (ps_parent_code >  $1 or  ps_group_code >= $2) and 
                (ps_parent_code >  $1 or  ps_group_code >  $2 or
ps_section_code >= $3) and 
                (ps_parent_code >  $1 or  ps_group_code >  $2 or
ps_section_code >  $3 or  ps_seq_no >  $4) 
        order by ps_parent_code, ps_group_code, ps_section_code,
        limit $5

Aside: this is the long way of writing
select 1::int4, * from data1.product_structure_file where
(ps_parent_code, ps_group_code, ps_section_code, ps_seq_no) > ($1, $2,
$3, $4) limit %5

which is allowed in pg but returns the wrong answer.


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