mysql-sql语句的查询优化，各位看看可以怎么优化，新人初学，SQL有点复杂，压力山大。。。。。

如下 项目用的是 mysql 是项目的查询SQL语句， 查询结果为 88 的竟然要1分40秒， 结果为5的要 2秒，
SQL语句不是我写的，大家看看有什么地方可以优化的，想要优化查询速度的话，可以从哪方面入手修改。

 SELECT   COUNT(DISTINCT ttsp.id) AS postpone_count,  ttp.task_document_id AS task_document_id,  ttm.id AS attention_id,  tt.company_id,  tt.create_by,  ts.name AS principal,  ts.staff_id AS principal_id,  tt.postpone_time,  tt.subtask_bind_step_id,  tt.current_task_step_id,  tt.create_time,  tts.task_step_id AS has_taskstep,  tts2.current_task_step,  tt.po_predict_work_load,  tt.task_priority,  tt.expect_end_time,  tts2.executor,  tt.start_time,  tt.end_time,  tt.type,  tta.task_annex_id AS annex,  tt.task_id AS id,  tt.title AS NAME,  CASE    WHEN (      (SELECT         COUNT(*)       FROM        t_task_step       WHERE task_id = tt.task_id         AND del_flag = 0) > 0    )     THEN 'closed'     WHEN (      (SELECT         COUNT(*)       FROM        t_task       WHERE pid = tt.task_id         AND del_flag = 0) > 0    )     THEN 'closed'     ELSE 'open'   END AS state,  '3' AS style,  tt.author AS task_author,  tt.work_load AS task_work_load,  tm.comment AS content_comment,  tn.comment AS time_comment,  tq.comment AS other_comment,  tnn.comment AS inner_comment,  CONCAT(tt.task_id, '_t') AS id_type,  SUM(tw.workload) count_workload,  tt.pid,  CASE    WHEN tt.task_priority = '1'     THEN 'task-emergency'     WHEN tt.task_priority = '2'     THEN 'task-imprtant'     WHEN tt.task_priority = '3'     THEN 'task-general'     ELSE 'task-normal'   END AS iconCls FROM  t_task tt   LEFT JOIN t_task_step tts     ON (      tt.task_id = tts.task_id       AND tts.del_flag = '0'    )   LEFT JOIN t_workload tw     ON (      tts.task_step_id = tw.task_step_id       AND tw.del_flag = '0'    )   LEFT JOIN t_task_content ttc     ON (      tt.task_id = ttc.task_id       AND ttc.del_flag = '0'    )   LEFT JOIN     (SELECT       *     FROM      t_task_annex     ORDER BY task_annex_id DESC) tta     ON (      tt.task_id = tta.task_id       AND tta.del_flag = '0'       AND tta.annex IS NOT NULL    )   LEFT JOIN t_project tp     ON tt.project_id = tp.project_id   LEFT JOIN t_company tco     ON tt.company_id = tco.company_id   LEFT JOIN     (SELECT       foreign_id,      COMMENT,      create_time     FROM      t_comment     WHERE del_flag = '0'       AND TYPE = 2       AND comment_type = 0     ORDER BY create_time DESC     LIMIT 0, 1) AS tm     ON tt.task_id = tm.foreign_id   LEFT JOIN     (SELECT       foreign_id,      COMMENT,      create_time     FROM      t_comment     WHERE del_flag = '0'       AND TYPE = 2       AND comment_type = 1     ORDER BY create_time DESC     LIMIT 0, 1) AS tn     ON tt.task_id = tn.foreign_id   LEFT JOIN     (SELECT       foreign_id,      COMMENT,      create_time     FROM      t_comment     WHERE del_flag = '0'       AND TYPE = 2       AND comment_type = 2     ORDER BY create_time DESC     LIMIT 0, 1) AS tq     ON tt.task_id = tq.foreign_id   LEFT JOIN     (SELECT       foreign_id,      COMMENT,      create_time     FROM      t_comment     WHERE del_flag = '0'       AND TYPE = 2       AND comment_type = 3     ORDER BY create_time DESC     LIMIT 0, 1) AS tnn     ON tt.task_id = tnn.foreign_id   LEFT JOIN     (SELECT       task_step_id,      TYPE AS current_task_step,      to_staff AS executor,      to_staff_email AS executor_email     FROM      t_task_step     WHERE del_flag = '0') tts2     ON tts2.task_step_id = tt.current_task_step_id   LEFT JOIN t_staff ts     ON (tt.executor = ts.staff_id)   LEFT JOIN     (SELECT       ttp.task_id,      GROUP_CONCAT(producttype_id SEPARATOR ',') AS producttype_id,      task_document_id     FROM      (SELECT         ttp.producttype_id,        ttp.task_id,        ttdl.task_document_id       FROM        t_task_producttype_link ttp         LEFT JOIN           (SELECT             producttype_id,            GROUP_CONCAT(id SEPARATOR ',') AS task_document_id           FROM            t_task_document_type_link           GROUP BY producttype_id) ttdl           ON ttp.producttype_id = ttdl.producttype_id) ttp     GROUP BY task_id) ttp     ON ttp.task_id = tt.task_id   LEFT JOIN t_task_step_postpone_history ttsp     ON ttsp.foreign_id = tt.task_id     AND ttsp.type = 0     AND ttsp.del_flag = 0   LEFT JOIN     (SELECT       *     FROM      t_task_myattention     WHERE user_id = '202b293f-da58-4cd3-b12d-40e4f9ce0d2c') ttm     ON ttm.task_id = tt.task_id WHERE tco.del_flag = '0'   AND tt.del_flag = '0'   AND tp.del_flag = '0'   AND tt.project_id = 44   AND tt.pid IS NULL GROUP BY tt.task_id ORDER BY tt.task_id DESC 

这是 explain的查询结果