Question

我有一个为MySQL编写的查询，它应该在多个表中搜索特定的数据片段，然后拉出一个“ReferenceID”来引用一个包含原始文档详细信息的主表。我可以快速搜索这些项目，但是当我尝试从主表中提取特定内容时，它似乎只是捶打磁盘而不是仅仅检索数据。

我想要做的查询看起来像这样：

SELECT ReferenceID as refid, Title as title, Contents as content, TimeSecs as age 
            FROM tabledata u
            WHERE u.ReferenceID IN (SELECT DISTINCT a.ReferenceID as refid FROM table1 a WHERE a.Table1Item = "[item]"
UNION SELECT DISTINCT b.ReferenceID as refid FROM table2 b WHERE b.Table2Item = "[item]"
UNION SELECT DISTINCT c.ReferenceID as refid FROM table3 c WHERE c.Table3Item = "[item]") 
ORDER BY TimeSecs DESC LIMIT 50

ReferenceID搜索中的SELECT查询也可能包含多个OR语句，但无论如何，这样的查询可能需要20-30秒，即使三个表中的查询搜索可能需要半秒钟。 / p>

此外，更令人困惑的是它使用filesort来浏览“tabledata”表，即使对于其他每个查询都没有。还为其适当的列和表创建了索引。

我输出了一个解释来证明这一点：

+----+------------------+--------------+----+----------------------+-----------+-------+-----+------+----------------------------+
|id  |select_type       |table         |type|possible_keys         |key        |key_len|ref  |rows  |Extra                       |
+----+------------------+--------------+----+----------------------+-----------+-------+-----+------+----------------------------+
|1   |PRIMARY           |u             |ALL |NULL                  |NULL       |NULL   |NULL |659659|Using where; Using filesort |
|2   |DEPENDENT SUBQUERY|a             |ref |ReferenceID,Table1Item|ReferenceID|98     |func |3     |Using where; Using temporary|
|3   |DEPENDENT UNION   |b             |ref |ReferenceID,Table2Item|Table2Item |386    |const|14    |Using where; Using temporary|
|4   |DEPENDENT UNION   |c             |ref |ReferenceID           |ReferenceID|98     |func |11    |Using where; Using temporary|
|5   |DEPENDENT UNION   |d             |ref |ReferenceID           |ReferenceID|98     |func |7     |Using where; Using temporary|
|NULL|UNION RESULT      |<union2,3,4,5>|ALL |NULL                  |NULL       |NULL   |NULL |NULL  |                            |
+----+------------------+--------------+----+----------------------+-----------+-------+-----+------+----------------------------+

总的来说，我确实得到了我想要的结果，但没有得到及时的结果。

Answer 1

您对查询的表达方式，SQL引擎可能会生成整个参考ID列表，然后开始查看它们。

请改为尝试：

SELECT ReferenceID as refid, Title as title, Contents as content, TimeSecs as age 
FROM tabledata u
WHERE exists (SELECT 1
              FROM table1 a
              WHERE a.Table1Item = "[item]" and u.ReferenceID = a.ReferenceID
             ) or
      exists (SELECT 1
              FROM table2 b
              WHERE b.Table2Item = "[item]" and u.ReferenceID = b.ReferenceID
             ) or
      exists (SELECT 1
              FROM table3 c
              WHERE c.Table3Item = "[item]" and u.ReferenceID = c.ReferenceID
             ) 
ORDER BY TimeSecs DESC
LIMIT 50;

然后确保在三个表中的每个表上都有ReferenceID, TablexItem的索引。

在多个表中快速搜索特定数据，然后引用主表

1 个答案: