Improve pruning of a default partition

When querying a partitioned table containing a default partition, we
were wrongly deciding to include it in the scan too early in the
process, failing to exclude it in some cases.  If we reinterpret the
PruneStepResult.scan_default flag slightly, we can do a better job at
detecting that it can be excluded.  The change is that we avoid setting
the flag for that pruning step unless the step absolutely requires the
default partition to be scanned (in contrast with the previous
arrangement, which was to set it unless the step was able to prune it).
So get_matching_partitions() must explicitly check the partition that
each returned bound value corresponds to in order to determine whether
the default one needs to be included, rather than relying on the flag
from the final step result.

Author: Yuzuko Hosoya <hosoya.yuzuko@lab.ntt.co.jp>
Reviewed-by: Amit Langote <Langote_Amit_f8@lab.ntt.co.jp>
Discussion: https://postgr.es/m/00e601d4ca86$932b8bc0$b982a340$@lab.ntt.co.jp

src/test/regress/expected/partition_prune.out

@@ -504,15 +504,13 @@ explain (costs off) select * from rlp where a <= 31;
          Filter: (a <= 31)
    ->  Seq Scan on rlp5_1
          Filter: (a <= 31)
-   ->  Seq Scan on rlp5_default
-         Filter: (a <= 31)
    ->  Seq Scan on rlp_default_10
          Filter: (a <= 31)
    ->  Seq Scan on rlp_default_30
          Filter: (a <= 31)
    ->  Seq Scan on rlp_default_default
          Filter: (a <= 31)
-(29 rows)
+(27 rows)
 
 explain (costs off) select * from rlp where a = 1 or a = 7;
            QUERY PLAN           
@@ -559,11 +557,7 @@ explain (costs off) select * from rlp where a > 20 and a < 27;
          Filter: ((a > 20) AND (a < 27))
    ->  Seq Scan on rlp4_2
          Filter: ((a > 20) AND (a < 27))
-   ->  Seq Scan on rlp4_default
-         Filter: ((a > 20) AND (a < 27))
-   ->  Seq Scan on rlp_default_default
-         Filter: ((a > 20) AND (a < 27))
-(9 rows)
+(5 rows)
 
 explain (costs off) select * from rlp where a = 29;
         QUERY PLAN        
@@ -588,6 +582,16 @@ explain (costs off) select * from rlp where a >= 29;
          Filter: (a >= 29)
 (11 rows)
 
+explain (costs off) select * from rlp where a < 1 or (a > 20 and a < 25);
+                      QUERY PLAN                      
+------------------------------------------------------
+ Append
+   ->  Seq Scan on rlp1
+         Filter: ((a < 1) OR ((a > 20) AND (a < 25)))
+   ->  Seq Scan on rlp4_1
+         Filter: ((a < 1) OR ((a > 20) AND (a < 25)))
+(5 rows)
+
 -- redundant clauses are eliminated
 explain (costs off) select * from rlp where a > 1 and a = 10;	/* only default */
             QUERY PLAN            

src/test/regress/sql/partition_prune.sql

@@ -83,6 +83,7 @@ explain (costs off) select * from rlp where a = 1 or b = 'ab';
 explain (costs off) select * from rlp where a > 20 and a < 27;
 explain (costs off) select * from rlp where a = 29;
 explain (costs off) select * from rlp where a >= 29;
+explain (costs off) select * from rlp where a < 1 or (a > 20 and a < 25);
 
 -- redundant clauses are eliminated
 explain (costs off) select * from rlp where a > 1 and a = 10;	/* only default */