Problem with default partition pruning

Hi,

I found the bug of default partition pruning when executing a range query.

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postgres=# create table test1(id int, val text) partition by range (id); 
postgres=# create table test1_1 partition of test1 for values from (0) to (100); 
postgres=# create table test1_2 partition of test1 for values from (150) to (200);
postgres=# create table test1_def partition of test1 default; 

postgres=# explain select * from test1 where id > 0 and id < 30;
                           QUERY PLAN                           
----------------------------------------------------------------
 Append  (cost=0.00..11.83 rows=59 width=11)
   ->  Seq Scan on test1_1  (cost=0.00..5.00 rows=58 width=11)
         Filter: ((id > 0) AND (id < 30))
   ->  Seq Scan on test1_def  (cost=0.00..6.53 rows=1 width=12)
         Filter: ((id > 0) AND (id < 30))
(5 rows)

There is no need to scan the default partition, but it's scanned.
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In the current implement, whether the default partition is scanned
or not is determined according to each condition of given WHERE
clause at get_matching_range_bounds().  In this example, scan_default
is set true according to id > 0 because id >= 200 matches the default
partition.  Similarly, according to id < 30, scan_default is set true.
Then, these results are combined according to AND/OR at perform_pruning_combine_step().
In this case, final result's scan_default is set true.

The modifications I made are as follows:
- get_matching_range_bounds() determines only offsets of range bounds
  according to each condition 
- These results are combined at perform_pruning_combine_step()
- Whether the default partition is scanned or not is determined at 
  get_matching_partitions()

Attached the patch.  Any feedback is greatly appreciated.

Best regards,
---
Yuzuko Hosoya
NTT Open Source Software Center