Description:
The detail is as follow.
OS version and name:
Ubuntu 22.04.3 LTS (Jammy Jellyfish)
Linux eb1f47b08982 6.5.11-8-pve #1 SMP PREEMPT_DYNAMIC PMX 6.5.11-8 (2024-01-30T12:27Z) x86_64 x86_64 x86_64 GNU/Linux
After executing some sql statements on multiple nodes, I found that executing the same SQL statement for querying metadata on a different node, which should give the same results, yielded very different results:
Poc:
```
SHOW INDEX FROM mytest100.test2;
```
The results on node 1 are as follows:
```
mysql> SHOW INDEX FROM mytest100.test2;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| test2 | 0 | PRIMARY | 1 | column0 | A | 24 | NULL | NULL | | BTREE | | | YES | NULL |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
1 row in set (0.00 sec)
```
The results on node 2 are as follows:
```
mysql> SHOW INDEX FROM mytest100.test2;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| test2 | 0 | PRIMARY | 1 | column0 | A | 14 | NULL | NULL | | BTREE | | | YES | NULL |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
1 row in set (0.00 sec)
```
I don't understand why there are different query results for metadata that should logically be perfectly consistent across nodes.
How to repeat:
Architecture Information:
'''
[NDBD DEFAULT]
NoOfReplicas =2
DataMemory = 512M
IndexMemory = 64M
[NDB_MGMD]
NodeId=1
hostname =192.172.10.8
datadir =/var/lib/mysql-cluster
[NDBD]
NodeId =2
hostname =192.172.10.9
datadir =/usr/local/mysql-cluster/data
NodeGroup=0
[NDBD]
NodeId =3
hostname =192.172.10.10
datadir =/usr/local/mysql-cluster/data
NodeGroup=1
[NDBD]
NodeId =4
hostname =192.172.10.11
datadir =/usr/local/mysql-cluster/data
NodeGroup=0
[NDBD]
NodeId =5
hostname =192.172.10.12
datadir =/usr/local/mysql-cluster/data
NodeGroup=1
[mysqld]
NodeId =6
hostname =192.172.10.9
[mysqld]
NodeId =7
hostname =192.172.10.10
[mysqld]
NodeId =8
hostname =192.172.10.11
[mysqld]
NodeId =9
hostname =192.172.10.12
'''
Suggested fix:
The scope of this bug can be quite serious. As you can see, the content of the Cardinality column is not consistent across different nodes, and Cardinality will directly affect the optimisation of statements and the selection of query plans, which will lead to the performance of sql on different nodes, and even the logical behaviour will be affected to varying degrees.
