WARNING: Query error: SSL SYSCALL error: EOF detected

While the pgagent job was doing a reindex on tbl1, another query tried to select from the same table that the job was working on.Then job failed with this error:

`WARNING: Query error: SSL SYSCALL error: EOF detected`

1. First the job was doing the reindex on the table, why the coming query did not go to wait state until the job finish?
2. How to solve the issue?