show `CARTESIAN JOIN` inappropriately in some cases

Description

mysql> desc select /*+ tidb_hj(t1) */ * from t t1 inner join t t2 where t1.a=t2.a and t1.a=1;
+---------------------------+---------+------+---------------+----------------------+
| id                        | estRows | task | access object | operator info        |
+---------------------------+---------+------+---------------+----------------------+
| HashJoin_7                | 1.00    | root |               | CARTESIAN inner join |
| ├─Point_Get_10(Build)     | 1.00    | root | table:t       | handle:1             |
| └─Point_Get_9(Probe)      | 1.00    | root | table:t       | handle:1             |
+---------------------------+---------+------+---------------+----------------------+
3 rows in set (0.00 sec)

Contributor guide

Tech stack

gosql

Domain

backenddatabase

Issue type

bug

DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.

2

Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.

half day

Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.

blocked

ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.

clear

Prerequisites

Go programming languageTiDB query executionSQL join concepts

Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.

30

Research direction

Investigate the code that formats the 'operator info' for HashJoin in the query plan. Look at how the join type is determined, especially when there are equal conditions. The issue occurs when a join with conditions is incorrectly labeled as CARTESIAN. Check the planner core package for the String method of Join operators.