[IOTDB-2032] Incorrect result of descending query by multi readers - ASF JIRA

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Details

Type: Bug
Status: Closed
Priority: Major
Resolution: Fixed
Affects Version/s: None
Fix Version/s: master branch, 0.12.4
Component/s: None
Labels:
- pull-request-available

Description

By replaying IoTDBTimePartitionIT.testOrderByTimeDesc

Steps:

Setup a 3 nodes cluster with 2 replicas
Enable time partition with partition_interval 2592000
Prepare the following data

insert into root.group_1.d_1(timestamp, s_1) values(2018-07-18T00:00:00.000+08:00, 18.0);
insert into root.group_1.d_1(timestamp, s_1) values(2018-07-19T00:00:00.000+08:00, 19.0);
insert into root.group_1.d_1(timestamp, s_1) values(2019-08-19T00:00:00.000+08:00, 20.0);

By executing query

select * from root.group_1.d_1 order by time desc;

We got

The result order is incorrect.

The main reason is:

The underlying data has been separated into different partitions, so we use two readers to read them. As we specify the decending order, the reader returns the data in a descending manner. However, in current cluster query, the AssignPathPriorityMergeReader merges the data from mutl readers with a min heap. But we need to use a max heap in a decending query.

reader1 returns [2019-08-19, 2018-07-19]
reader2 returns [2018-07-18]
Use a min heap, it returns 2018-07-18 firstly.
and then reader2 is empty, it will return 2019-08-19, and 2018-07-19.

If we use a max heap instead, it will return 2019-08-19 firstly.
And reader1 has 2018-07-19 left, reader2 has 2018-07-18.
Then 2018-07-19 will be returned because of the max heap.
The last one is 2018-07-18.