last_value()/first_value() returns incorrect and unstable result

[evenyag]

What type of bug is this?

Incorrect result

What subsystems are affected?

Standalone mode

Minimal reproduce step

CREATE TABLE monitor (
  host STRING,
  ts TIMESTAMP DEFAULT CURRENT_TIMESTAMP() TIME INDEX,
  cpu FLOAT64 DEFAULT 0,
  memory FLOAT64,
  PRIMARY KEY(host)) ENGINE=mito;

INSERT INTO monitor
VALUES
    ("127.0.0.1", 1, 0.5, 0.9),
    ("127.0.0.2", 2, 0.6, 0.8),
    ("127.0.0.3", 3, 0.7, 0.7),
    ("127.0.0.4", 4, 0.8, 0.6),
    ("127.0.0.4", 5, 0.9, 0.5),
    ("127.0.0.5", 6, 1.0, 0.4);

select last_value(cpu order by ts) from monitor;

select first_value(cpu order by ts) from monitor;

What did you expect to see?

select last_value(cpu order by ts) from monitor;

+--------------------------------------------------------------+
| last_value(monitor.cpu) ORDER BY [monitor.ts ASC NULLS LAST] |
+--------------------------------------------------------------+
|                                                            1 |
+--------------------------------------------------------------+

select first_value(cpu order by ts) from monitor;

+---------------------------------------------------------------+
| first_value(monitor.cpu) ORDER BY [monitor.ts ASC NULLS LAST] |
+---------------------------------------------------------------+
|                                                           0.5 |
+---------------------------------------------------------------+

What did you see instead?

select last_value(cpu order by ts) from monitor;
+--------------------------------------------------------------+
| last_value(monitor.cpu) ORDER BY [monitor.ts ASC NULLS LAST] |
+--------------------------------------------------------------+
|                                                          0.6 |
+--------------------------------------------------------------+

select first_value(cpu order by ts) from monitor;
+---------------------------------------------------------------+
| first_value(monitor.cpu) ORDER BY [monitor.ts ASC NULLS LAST] |
+---------------------------------------------------------------+
|                                                           0.6 |
+---------------------------------------------------------------+

What operating system did you use?

Unrelated

What version of GreptimeDB did you use?

0.8.1

Relevant log output and stack trace

NA