Question

我想运行以下搜索：

schema->resultset('Entity')->search({ 
        -or => { "me.user_id" => $user_id, 'set_to_user.user_id' => $user_id } 
    }, {
        'distinct' => 1,
        'join' => {'entity_to_set' => {'entity_set' => 'set_to_user'}},
        'order_by' => {'-desc' => 'modified'},
        'page' => 1,'rows' => 100
    });

在包含表格的数据库中，如下所示。

CREATE TABLE entity (
  id varchar(500) NOT NULL,
  user_id varchar(100) NOT NULL,
  modified timestamp NOT NULL,
  PRIMARY KEY (id, user_id),
  FOREIGN KEY (user_id) REFERENCES user(id) ON DELETE CASCADE ON UPDATE CASCADE
);

CREATE TABLE entity_to_set (
  set_id varchar(100) NOT NULL,
  user_id varchar(100) NOT NULL,
  entity_id varchar(500) NOT NULL,
  PRIMARY KEY (set_id, user_id, entity_id),
  FOREIGN KEY (entity_id, user_id) REFERENCES entity(id, user_id) ON DELETE CASCADE ON UPDATE CASCADE,
  FOREIGN KEY (set_id) REFERENCES entity_set(id) ON DELETE CASCADE ON UPDATE CASCADE
);

CREATE TABLE entity_set (
  id varchar(100) NOT NULL,
  PRIMARY KEY (id)
);

CREATE TABLE set_to_user (
  set_id varchar(100) NOT NULL,
  user_id varchar(100) NOT NULL,
  PRIMARY KEY (set_id, user_id),
  FOREIGN KEY (user_id) REFERENCES user(id) ON DELETE CASCADE ON UPDATE CASCADE,
  FOREIGN KEY (set_id) REFERENCES entity_set(id) ON DELETE CASCADE ON UPDATE CASCADE
);

CREATE TABLE user (
  id varchar(100) NOT NULL,
  PRIMARY KEY (id)
);

我有大约6000 entity，6000 entity_to_set，10 entity_set和50 set_to_user。

现在，这个查询需要一些时间，（一两秒钟），这是不幸的。仅对实体表（包括ORDER BY）进行查询时，结果几乎是即时的。作为调试的第一步，我找到了DBIC代码变为的实际SQL查询：

SELECT me.id, me.user_id, me.modified FROM entity me
LEFT JOIN entity_to_set entity_to_set ON ( entity_to_set.entity_id = me.id AND entity_to_set.user_id = me.user_id ) 
LEFT JOIN entity_set entity_set ON entity_set.id = entity_to_set.set_id 
LEFT JOIN set_to_user set_to_user ON set_to_user.set_id = entity_set.id 
WHERE ( ( set_to_user.user_id = 'Craigy' OR me.user_id = 'Craigy' ) ) 
GROUP BY me.id, me.user_id, me.modified ORDER BY modified DESC LIMIT 100;

以下是EXPLAIN QUERY PLAN

的结果

0|0|0|SCAN TABLE entity AS me USING INDEX sqlite_autoindex_entity_1 (~1000000 rows)
0|1|1|SEARCH TABLE entity_to_set AS entity_to_set USING COVERING INDEX entity_to_set_idx_cover (entity_id=? AND user_id=?) (~9 rows)
0|2|2|SEARCH TABLE entity_set AS entity_set USING COVERING INDEX sqlite_autoindex_entity_set_1 (id=?) (~1 rows)
0|3|3|SEARCH TABLE set_to_user AS set_to_user USING COVERING INDEX sqlite_autoindex_set_to_user_1 (set_id=?) (~5 rows)
0|0|0|USE TEMP B-TREE FOR ORDER BY

其中entity_to_set_idx_cover是

CREATE INDEX entity_to_set_idx_cover ON entity_to_set (entity_id, user_id, set_id);

现在，问题是用于排序的b树，而不是在我不进行连接时使用的索引。

我注意到DBIx :: Class将'distinct' => 1转换为GROUP BY语句（I believe the documentation says they are equivalent here）。我删除了GROUP BY语句并改为使用SELECT DISTINCT，并使用以下查询

SELECT DISTINCT me.id, me.user_id, me.modified FROM entity me
LEFT JOIN entity_to_set entity_to_set ON ( entity_to_set.entity_id = me.id AND entity_to_set.user_id = me.user_id ) 
LEFT JOIN entity_set entity_set ON entity_set.id = entity_to_set.set_id 
LEFT JOIN set_to_user set_to_user ON set_to_user.set_id = entity_set.id 
WHERE ( ( set_to_user.user_id = 'Craigy' OR me.user_id = 'Craigy' ) ) 
ORDER BY modified DESC LIMIT 100;

我相信它会给出相同的结果。此查询的EXPLAIN QUERY PLAN为

0|0|0|SCAN TABLE entity AS me USING COVERING INDEX entity_sort_modified_user_id (~1000000 rows)
0|1|1|SEARCH TABLE entity_to_set AS entity_to_set USING COVERING INDEX entity_to_set_idx_cover (entity_id=? AND user_id=?) (~9 rows)
0|2|2|SEARCH TABLE entity_set AS entity_set USING COVERING INDEX sqlite_autoindex_entity_set_1 (id=?) (~1 rows)
0|3|3|SEARCH TABLE set_to_user AS set_to_user USING COVERING INDEX sqlite_autoindex_set_to_user_1 (set_id=?) (~5 rows)

其中entity_sort_modified_user_id是使用

创建的索引

CREATE INDEX entity_sort_modified_user_id ON entity (modified, user_id, id);

这几乎是瞬间运行（没有b树）。

编辑：为了证明当ORDER BY按升序排列时问题仍然存在，以及索引对这些查询的影响，这里是对相同表的类似查询。前两个查询分别没有使用SELECT DISTINCT和GROUP BY的索引，后两个查询使用相同的查询和索引。

sqlite> EXPLAIN QUERY PLAN SELECT DISTINCT me.id, me.user_id, me.modified FROM entity me LEFT JOIN entity_to_set entity_to_set ON ( entity_to_set.entity_id = me.id AND entity_to_set.user_id = me.user_id ) LEFT JOIN entity_set entity_set ON entity_set.id = entity_to_set.set_id WHERE ( me.user_id = 'Craigy' AND entity_set.id = 'SetID' ) ORDER BY modified LIMIT 100;
0|0|0|SCAN TABLE entity AS me (~100000 rows)
0|1|1|SEARCH TABLE entity_to_set AS entity_to_set USING AUTOMATIC COVERING INDEX (entity_id=? AND user_id=?) (~7 rows)
0|2|2|SEARCH TABLE entity_set AS entity_set USING COVERING INDEX sqlite_autoindex_entity_set_1 (id=?) (~1 rows)
0|0|0|USE TEMP B-TREE FOR DISTINCT
0|0|0|USE TEMP B-TREE FOR ORDER BY
sqlite> EXPLAIN QUERY PLAN SELECT me.id, me.user_id, me.modified FROM entity me LEFT JOIN entity_to_set entity_to_set ON ( entity_to_set.entity_id = me.id AND entity_to_set.user_id = me.user_id ) LEFT JOIN entity_set entity_set ON entity_set.id = entity_to_set.set_id WHERE ( me.user_id = 'Craigy' AND entity_set.id = 'SetID' ) GROUP BY me.id, me.user_id, me.modified ORDER BY modified LIMIT 100;
0|0|0|SCAN TABLE entity AS me USING INDEX sqlite_autoindex_entity_1 (~100000 rows)
0|1|1|SEARCH TABLE entity_to_set AS entity_to_set USING AUTOMATIC COVERING INDEX (entity_id=? AND user_id=?) (~7 rows)
0|2|2|SEARCH TABLE entity_set AS entity_set USING COVERING INDEX sqlite_autoindex_entity_set_1 (id=?) (~1 rows)
0|0|0|USE TEMP B-TREE FOR ORDER BY
sqlite> CREATE INDEX entity_idx_user_id_modified_id ON entity (user_id, modified, id);
sqlite> EXPLAIN QUERY PLAN SELECT DISTINCT me.id, me.user_id, me.modified FROM entity me LEFT JOIN entity_to_set entity_to_set ON ( entity_to_set.entity_id = me.id AND entity_to_set.user_id = me.user_id ) LEFT JOIN entity_set entity_set ON entity_set.id = entity_to_set.set_id WHERE ( me.user_id = 'Craigy' AND entity_set.id = 'SetID' ) ORDER BY modified LIMIT 100;
0|0|0|SEARCH TABLE entity AS me USING COVERING INDEX entity_idx_user_id_modified_id (user_id=?) (~10 rows)
0|1|1|SEARCH TABLE entity_to_set AS entity_to_set USING AUTOMATIC COVERING INDEX (entity_id=? AND user_id=?) (~7 rows)
0|2|2|SEARCH TABLE entity_set AS entity_set USING COVERING INDEX sqlite_autoindex_entity_set_1 (id=?) (~1 rows)
sqlite> EXPLAIN QUERY PLAN SELECT me.id, me.user_id, me.modified FROM entity me LEFT JOIN entity_to_set entity_to_set ON ( entity_to_set.entity_id = me.id AND entity_to_set.user_id = me.user_id ) LEFT JOIN entity_set entity_set ON entity_set.id = entity_to_set.set_id WHERE ( me.user_id = 'Craigy' AND entity_set.id = 'SetID' ) GROUP BY me.id, me.user_id, me.modified ORDER BY modified LIMIT 100;
0|0|0|SEARCH TABLE entity AS me USING COVERING INDEX entity_idx_user_id_modified_id (user_id=?) (~10 rows)
0|1|1|SEARCH TABLE entity_to_set AS entity_to_set USING AUTOMATIC COVERING INDEX (entity_id=? AND user_id=?) (~7 rows)
0|2|2|SEARCH TABLE entity_set AS entity_set USING COVERING INDEX sqlite_autoindex_entity_set_1 (id=?) (~1 rows)
0|0|0|USE TEMP B-TREE FOR GROUP BY
0|0|0|USE TEMP B-TREE FOR ORDER BY

我的问题是：如何修复我的DBIx :: Class代码，使其执行与SELECT DISTINCT查询一样好。或者我如何添加索引以使其工作正常？还是需要其他一些修复？

Answer 1

注意：这不是这个问题的完整答案。它仅显示在按升序顺序排序时如何避免临时b树。当需要按降序顺序排序时，AFAIK当前（版本3.8.1）没办法（没有调整sqlite）以避免GROUP BY版本的临时b树。

使用问题中的表定义和索引：

sqlite> select sqlite_version();
sqlite_version()
----------------
3.8.1

当（a）ORDER BY按升序排列并且（b）GROUP BY子句逐列匹配ORDER BY子句时，您的查询在没有临时b树的情况下运行。

除GROUP BY和ORDER BY子句外，查询保持不变：

/* table definitions as shown in the question */
sqlite> CREATE INDEX entity_to_set_idx_cover ON entity_to_set (entity_id, user_id, set_id);
sqlite> CREATE INDEX entity_sort_modified_user_id ON entity (modified, user_id, id);

sqlite> EXPLAIN QUERY PLAN
   ...> SELECT  me.id, me.user_id, me.modified FROM entity me
   ...> LEFT JOIN entity_to_set entity_to_set ON ( entity_to_set.entity_id = me.id AND entity_to_set.user_id = me.user_id )
   ...> LEFT JOIN entity_set entity_set ON entity_set.id = entity_to_set.set_id
   ...> LEFT JOIN set_to_user set_to_user ON set_to_user.set_id = entity_set.id
   ...> WHERE ( ( set_to_user.user_id = 'Craigy' OR me.user_id = 'Craigy' ) )
   ...> GROUP BY me.modified,  me.user_id, me.id
   ...> ORDER BY me.modified,  me.user_id, me.id ASC LIMIT 100;

selectid    order       from        detail
----------  ----------  ----------  -------------------------------------------------------------------------
0           0           0           SCAN TABLE entity AS me USING COVERING INDEX entity_sort_modified_user_id
0           1           1           SEARCH TABLE entity_to_set AS entity_to_set USING COVERING INDEX entity_t
0           2           2           SEARCH TABLE entity_set AS entity_set USING COVERING INDEX sqlite_autoind
0           3           3           SEARCH TABLE set_to_user AS set_to_user USING COVERING INDEX sqlite_autoi

但是，当你按降序排序时，你得到一个临时b树：

   ...> ...
   ...> GROUP BY me.modified,  me.user_id, me.id
   ...> ORDER BY me.modified,  me.user_id, me.id DESC LIMIT 100;
selectid    order       from        detail
----------  ----------  ----------  -------------------------------------------------------------------------
0           0           0           SCAN TABLE entity AS me USING COVERING INDEX entity_sort_modified_user_id
0           1           1           SEARCH TABLE entity_to_set AS entity_to_set USING COVERING INDEX entity_t
0           2           2           SEARCH TABLE entity_set AS entity_set USING COVERING INDEX sqlite_autoind
0           3           3           SEARCH TABLE set_to_user AS set_to_user USING COVERING INDEX sqlite_autoi
0           0           0           USE TEMP B-TREE FOR ORDER BY

原因是sqlite（直到当前版本的3.8.1）无法识别它可以按降序进行分组。因此，您将始终获得单独的步骤。即使将索引声明为DESC，也无法避免这种情况。请参阅有关sqlite mailing list的讨论。

<强>结论如果您希望查询ORDER BY DESC而不使用临时b树，则必须调整SQL生成以使用DISTINCT。

选择明显比group by快

1 个答案: