如何使用 NOT EXISTS MySQL 修复缓慢的查询性能？ CodeIgniter

Question

我正在尝试优化/加快数据加载的性能，有没有办法解决这个问题？

我有3张桌子：

tbl_tt_college_studentpersonalinfo - 此表显示学生的姓名，加入我的查询

tbl_tt_college_preenrollment - 此表显示预注册学生的待定状态

tbl_tt_college_enlistment - 此表将检查学生的现有记录。

这是我的查询： - 我遇到的问题是数据加载速度很慢，可能需要 30-50 秒。 有没有办法改进它或使它像一个连接查询？

 public function get_enlists() { $this->db->join('tbl_tt_college_studentpersonalinfo','tbl_tt_college_preenrollment.studentID = tbl_tt_college_studentpersonalinfo.studentID'); $this->db->where('tbl_tt_college_preenrollment.departmentID', $this->input->post('course')); $this->db->where('tbl_tt_college_preenrollment.yearLevel', $this->input->post('yearLevel')); $this->db->where("NOT EXISTS (SELECT tbl_tt_college_enlistment.studentKeyID FROM tbl_tt_college_enlistment WHERE tbl_tt_college_enlistment.studentKeyID = tbl_tt_college_preenrollment.studentKeyID AND schoolYear = '". $this->session->userdata('currentAcademicYear'). '-'. ($this->session->userdata('currentAcademicYear') + 1). "' AND semester = '". $this->session->userdata('currentSemester'). "')"); $query = $this->db->get('tbl_tt_college_preenrollment'); return $query->result_array(); }

这是参考的原始查询：

 NOT EXISTS ( SELECT studentKeyID FROM tbl_tt_college_enlistment WHERE tbl_tt_college_studentpersonalinfo.studentKeyID = tbl_tt_college_enlistment.studentKeyID AND schoolYear = '". $this->session->userdata('currentAcademicYear'). '-'. ($this->session->userdata('currentAcademicYear') + 1). "' AND semester = '". $this->session->userdata('currentSemester'). "' )

Answer 1

See if this composite index on tbl_tt_college_enlistment helps:

INDEX(schoolYear, semester, studentKeyID)

If not adequate, please provide SHOW CREATE TABLE and EXPLAIN SELECT ...