Re: [HACKERS] Gather Merge

I am sorry for the delay, here is the latest re-based patch.

my colleague Neha Sharma, reported one regression with the patch, where

explain output for the Sort node under GatherMerge was always showing

cost as zero:

explain analyze select '' AS "xxx" from pgbench_accounts  where filler like '%foo%' order by aid;
                                                                   QUERY PLAN                                                                  
------------------------------------------------------------------------------------------------------------------------------------------------
 Gather Merge  (cost=47169.81..70839.91 rows=197688 width=36) (actual time=406.297..653.572 rows=200000 loops=1)
   Workers Planned: 4
   Workers Launched: 4
   ->  Sort  (cost=0.00..0.00 rows=0 width=0) (actual time=368.945..391.124 rows=40000 loops=5)
         Sort Key: aid
         Sort Method: quicksort  Memory: 3423kB
         ->  Parallel Seq Scan on pgbench_accounts  (cost=0.00..42316.60 rows=49422 width=36) (actual time=296.612..338.873 rows=40000 loops=5)
               Filter: (filler ~~ '%foo%'::text)
               Rows Removed by Filter: 360000
 Planning time: 0.184 ms
 Execution time: 734.963 ms

This patch also fix that issue.