index disabled when using variable

MySQL & MariaDB Percona XtraDB Cluster 5.x
1

Hi all :slight_smile:
When using the stored procedure, I passed the cursor to the SQL and found that the SQL was unable to use the index.

You can reproduce this when using session variables,For example

Correct use index：
1.SELECT count(1) FROM T_E4S_BU_ECA_FAIL_UNCONT where FAIL_NO=‘H2923LFR001690’ ;

Index disabled，full table scan：
2.set @FAIL_NO=‘H2923LFR001690’ ;
SELECT count(1) FROM T_E4S_BU_ECA_FAIL_UNCONT where FAIL_NO=@FAIL_NO ;

How can i do? thx