如何提高 MySQL 中嵌套 SELECT 语句的性能

Question

我有以下性能很差的 SQL 语句：

SELECT  
frmInstLastModifiedDate AS last_modified 
, frmInstID AS proj_id 
, frmInstIsApproved 
, frmInstStatus AS proj_sts 
, (CASE 
    WHEN frmInstCreator = 294  THEN 'M' 
    WHEN status = 'f'  THEN 'F' 
    WHEN (frmInstID IN (SELECT shr.frmInstID from tbl_frm_share as shr WHERE shr.shrMember = 294 AND shr.shrType = 'LIKE'  AND shr.frmID = inst.frmID)) THEN 'L' 
    WHEN (frmInstID IN (SELECT shr.frmInstID from tbl_frm_share as shr WHERE shr.shrMember = 294 AND shr.shrType = 'SHARE' AND shr.frmID = inst.frmID)) THEN 'S' 
    ELSE 'O' END) as proj_grp 
, (SELECT lkpCode FROM tbl_frm_lookup WHERE lkpID = (SELECT ansValue FROM tbl_itm_answer WHERE frmInstID = proj_id AND itmID = (select itmID from tbl_frm_item where itmName = 'prjStatus' AND frmID = inst.frmID))) as prjStatus 
, (SELECT lkpCode FROM tbl_frm_lookup WHERE lkpID = (SELECT ansValue FROM tbl_itm_answer WHERE frmInstID = proj_id AND itmID = (select itmID from tbl_frm_item where itmName = 'ProjectType' AND frmID = inst.frmID))) as ProjectType 
, (SELECT itmID FROM tbl_itm_answer where itmID in (828,829,830,831) and frmInstID = proj_id AND SUBSTRING(ansValue,1,2) = 'on') as primIRWMObj 
, (SELECT lkpCode FROM tbl_frm_lookup WHERE lkpID = (SELECT ansValue FROM tbl_itm_answer WHERE frmInstID = proj_id AND itmID = (select itmID from tbl_frm_item where itmName = 'PrjPSubReg'))) as ProjectSubReg 
, frmInstCreator AS proj_creatorID 
, frmInstCode AS proj_code 
    FROM tbl_frm_instance inst 
WHERE status not like 'd'   
HAVING (proj_sts like 'c' AND ('PROJECT PROPONENT' = 'ADMIN' or proj_creatorID = 294 )) 
    or (proj_sts like 'a') 
    or (proj_sts like 't' AND proj_creatorID = 294)  
    OR (proj_grp = 'S')  
ORDER BY frmInstCreateDate DESC ;

语句是根据用户选择的选项动态创建的。我知道嵌套的 select 语句（如下所示）是问题所在，但我不知道如何替换它

    (SELECT lkpCode FROM tbl_frm_lookup WHERE lkpID = (SELECT ansValue FROM tbl_itm_answer WHERE frmInstID = proj_id AND itmID = (select itmID from tbl_frm_item where itmName = 'prjStatus' AND frmID = inst.frmID))) as prjStatus 

任何帮助将不胜感激。

Answer 1

不要使用nested selects，它最慢，而且数据库写得很脏。最好的方法是使用JOIN，尽可能使用JOIN。

如果您想创建具有良好性能的IS（这是您的工作，您的名字），那么您必须始终决定最有效、最快速、最安全的方法，那就是JOIN。

我建议你在任何地方都使用JOIN。

我建议您阅读Understanding the Query Execution Plan。

注意：有时您必须认为“像数据库”而不是“像过程”。

Answer 2

试试这个::

我能做的最少::

SELECT  
frmInstLastModifiedDate AS last_modified 
, frmInstID AS proj_id 
, frmInstIsApproved 
, frmInstStatus AS proj_sts 
, (CASE 
    WHEN frmInstCreator = 294  THEN 'M' 
    WHEN status = 'f'  THEN 'F' 
    WHEN (frmInstID IN (SELECT shr.frmInstID from tbl_frm_share as shr WHERE shr.shrMember = 294 AND shr.shrType = 'LIKE'  AND shr.frmID = inst.frmID)) THEN 'L' 
    WHEN (frmInstID IN (SELECT shr.frmInstID from tbl_frm_share as shr WHERE shr.shrMember = 294 AND shr.shrType = 'SHARE' AND shr.frmID = inst.frmID)) THEN 'S' 
    ELSE 'O' END) as proj_grp 
, 
(SELECT lkpCode 
FROM tbl_frm_lookup tfl
inner join tbl_itm_answer tia on (tfl.lkpID= tia.ansValue)
inner join tbl_frm_item tfi on (tia.itmID= tfi.itmID)
 WHERE frmInstID = proj_id and itmName = 'prjStatus' AND frmID = inst.frmID) as prjStatus 
, frmInstCreator AS proj_creatorID 
, frmInstCode AS proj_code