Hi,
Thanks for finding the bug.
The issue here is that in case of partitioned table, "estate->es_result_relation_info" is not pointing to the correct resultRelInfo. It is actually pointing to the last partition rather than the partition whose buffer is being flushed. For e.g. consider the following case.
create table part_tab (a int primary key, b text) partition by range (a);
create table part_tab_1 partition of part_tab for values from (1) to (2);
create table part_tab_2 partition of part_tab for values from (2) to (3);
create table part_tab_3 partition of part_tab for values from (3) to (4);
insert into part_tab values (1, 'str1'), (2, 'str2'), (3, 'str3');
copy (select * from part_tab) to '/tmp/multi_insert_part_tab.csv' csv;
truncate table part_tab;
copy part_tab from '/tmp/multi_insert_part_tab.csv' csv;
When above COPY FROM command is executed into the partitioned table (part_tab), for the first record i.e. (1, 'str1') 'estate->es_result_relation_info' gets updated with the resultRelInfo of partition-1 as the first record fits into partition 1. Similarly, for the second record, 'estate->es_result_relation_info' gets updated with the resultRelInfo of partition-2 and finally for the last record (i.e. (3, 'str3')), 'estate->es_result_relation_info' gets updated with the resultRelInfo of partition-3. Eventually, when all the records are read and the buffers are flushed one by one, we also do the index insertion (as there exists an index on the partitons) but during index insertion, we refer to the resultRelInfo in estate which is actually pointing to the last partition i.e. partition-3 in our case.
During heap insertion we actually refer to buffer->resultRelInfo which is always updated and that's the reason heap insertion works fine but not the index insertion.
Attached patch fixes the issue. It basically updates estate->es_result_relation_info with the correct resultRelInfo in CopyMultiInsertBufferFlush(). I've also added the test-case for it.
Andres, David, do you all agree with above analysis and the proposed fix ?