MySQL EXPLAIN 实践汇总

标签：EXPLAIN 汇总 tabname key MySQL NULL type id select

MySQLEXPLAIN可以显示估计查询语句执行计划，从中可以分析查询的执行情况是不是最优，这有助于我们对不使用索引的查询进行优化。EXPLAIN对于每个查询语句返回一行信息，它列出了一个有序的表格，MySQL处理语句的时候会读取他们。MySQL解决所有的连接都使用嵌套连接方法。这意味着MySQL读取第一张一行，然后匹配第二张表的所有行，第三张表或更多表都如此。当所有的表在处理时，MySQL会输出已经查询出来的列，并且回溯到表继续查找直到所有的行被找到，从该表读取下一行，直到程序继续处理下一张表。

当使用关键词 EXTENDED 时，EXPLAIN 会处理通过 SHOW WARNINGS 看到的一些而外信息。EXPLAIN EXTENDED 也会显示这些滤出的列。

语法：

EXPLAIN <select statement>;

输出表格字段如下：

mysql> explain select * from mysql.user where user='root';
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | user  | ALL  | NULL          | NULL | NULL    | NULL |    4 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+

Column	JSONName	Meaning
`id`	`select_id`	查询标识。id越大优先执行；id相同自上而下执行；
`select_type`	None	查询的类型
`table`	`table_name`	查询的表
`partitions`	`partitions`	Thematching partitions
`type`	`access_type`	连接类型
`possible_keys`	`possible_keys`	可能选择的索引
`key`	`key`	实际使用的索引
`key_len`	`key_length`	使用的索引长度
`ref`	`ref`	哪一列或常数在查询中与索引键列一起使用
`rows`	`rows`	估计查询的行数
`filtered`	`filtered`	被条件过滤掉的行数百分比
`Extra`	None	解决查询的一些额外信息

以下主要举例说明3个字段：select_type 、type、Extra

【select_type】

alue	JSONName	Meaning
`SIMPLE`	None	`简单查询` (不使用`UNION`或子查询)
`PRIMARY`	None	`外层查询，主查询`
`UNION`	None	`UNION``中`第二个语句或后面的语句
`DEPENDENTUNION`	`dependent` (`true`)	`UNION``中第二个语句或后面的语句`,独立于外部查询
`UNIONRESULT`	`union_result`	`UNION``的结果`
`SUBQUERY`	None	子查询中第一个SELECT
`DEPENDENTSUBQUERY`	`dependent` (`true`)	子查询中第一个SELECT,独立于外部查询
`DERIVED`	None	子查询在 FROM子句中
`MATERIALIZED`	`materialized_from_subquery`	物化子查询（不清楚是什么样的查询语句？）
`UNCACHEABLESUBQUERY`	`cacheable` (`false`)	结果集不能被缓存的子查询，必须重新评估外层查询的每一行
`UNCACHEABLEUNION`	`cacheable` (`false`)	`UNION``中第二个语句或后面的语句属于不可缓存的子查询`

创建测试表：

create table tabname (
id int auto_increment not null primary key,
name varchar(10) null,
indate datetime null,
tid int null,
key(tid),
key(indate)
)engine=innodb;


create table tabname2 (
id int auto_increment not null primary key,
name varchar(10) null,
indate datetime null,
tid int null,
key(tid),
key(indate)
)engine=myisam;


insert into tabname(name,indate,tid) values('love',now(),2),('lucky',now(),3),('passion',now(),4);
insert into tabname2(name,indate,tid) values('love',now(),2),('lucky',now(),3),('passion',now(),4);

#SIMPLE

mysql> explain select * from tabname;
+----+-------------+---------+------+---------------+------+---------+------+------+-------+
| id | select_type | table   | type | possible_keys | key  | key_len | ref  | rows | Extra |
+----+-------------+---------+------+---------------+------+---------+------+------+-------+
|  1 | SIMPLE      | tabname | ALL  | NULL          | NULL | NULL    | NULL |    3 |       |
+----+-------------+---------+------+---------------+------+---------+------+------+-------+

#PRIMARY / DERIVED

mysql> explain select * from (select * from tabname) as a;
+----+-------------+------------+------+---------------+------+---------+------+------+-------+
| id | select_type | table      | type | possible_keys | key  | key_len | ref  | rows | Extra |
+----+-------------+------------+------+---------------+------+---------+------+------+-------+
|  1 | PRIMARY     | <derived2> | ALL  | NULL          | NULL | NULL    | NULL |    3 |       |
|  2 | DERIVED     | tabname    | ALL  | NULL          | NULL | NULL    | NULL |    3 |       |
+----+-------------+------------+------+---------------+------+---------+------+------+-------+

#PRIMARY / UNION / UNION RESULT

mysql> explain select * from tabname union select * from tabname;
mysql> explain select * from tabname union all select * from tabname;
+----+--------------+------------+------+---------------+------+---------+------+------+-------+
| id | select_type  | table      | type | possible_keys | key  | key_len | ref  | rows | Extra |
+----+--------------+------------+------+---------------+------+---------+------+------+-------+
|  1 | PRIMARY      | tabname    | ALL  | NULL          | NULL | NULL    | NULL |    3 |       |
|  2 | UNION        | tabname    | ALL  | NULL          | NULL | NULL    | NULL |    3 |       |
| NULL | UNION RESULT | <union1,2> | ALL  | NULL          | NULL | NULL    | NULL | NULL |       |
+----+--------------+------------+------+---------------+------+---------+------+------+-------+

#PRIMARY / SUBQUERY

mysql> explain select * from tabname where id=(select max(id) from tabname);
+----+-------------+---------+-------+---------------+---------+---------+-------+------+------------------------------+
| id | select_type | table   | type  | possible_keys | key     | key_len | ref   | rows | Extra                        |
+----+-------------+---------+-------+---------------+---------+---------+-------+------+------------------------------+
|  1 | PRIMARY     | tabname | const | PRIMARY       | PRIMARY | 4       | const |    1 |                              |
|  2 | SUBQUERY    | NULL    | NULL  | NULL          | NULL    | NULL    | NULL  | NULL | Select tables optimized away |
+----+-------------+---------+-------+---------------+---------+---------+-------+------+------------------------------+

#PRIMARY / DEPENDENT SUBQUERY

mysql> explain select * from tabname a where exists(select 1 from tabname b where a.id=b.id);
mysql> explain select *,(select name from tabname b where a.id=b.id) from tabname a;
mysql> explain select * from tabname where id not in(select id from tabname);
+----+--------------------+---------+-----------------+---------------+---------+---------+------+------+-------------+
| id | select_type        | table   | type            | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+--------------------+---------+-----------------+---------------+---------+---------+------+------+-------------+
|  1 | PRIMARY            | tabname | ALL             | NULL          | NULL    | NULL    | NULL |    3 | Using where |
|  2 | DEPENDENT SUBQUERY | tabname | unique_subquery | PRIMARY       | PRIMARY | 4       | func |    1 | Using index |
+----+--------------------+---------+-----------------+---------------+---------+---------+------+------+-------------+

#PRIMARY / DEPENDENT UNION / DEPENDENT SUBQUERY / UNION RESULT

mysql> explain select * from tabname where id in (select id from tabname union select id from tabname);
+----+--------------------+------------+--------+---------------+---------+---------+------+------+-------------+
| id | select_type        | table      | type   | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+--------------------+------------+--------+---------------+---------+---------+------+------+-------------+
|  1 | PRIMARY            | tabname    | ALL    | NULL          | NULL    | NULL    | NULL |    3 | Using where |
|  2 | DEPENDENT SUBQUERY | tabname    | eq_ref | PRIMARY       | PRIMARY | 4       | func |    1 | Using index |
|  3 | DEPENDENT UNION    | tabname    | eq_ref | PRIMARY       | PRIMARY | 4       | func |    1 | Using index |
| NULL| UNION RESULT      | <union2,3> | ALL    | NULL          | NULL    | NULL    | NULL | NULL |             |
+----+--------------------+------------+--------+---------------+---------+---------+------+------+-------------+

【type】

type	Meaning
`system`	表仅一行数据 (=system table).这是const连接类型的特例。
`const`	`表最多只有一个匹配行，在查询开始时被读取。因为只有一个值，优化器将该列值视为常量。当在``primarykey``或者``unique``索引作为常量比较时被使用。`
`eq_ref(engine=myisam)`	`来自前面表的结果集中读取一行，这是除``system``和``const``外最好的连接类型。当在使用``PRIMARYKEY``或者``UNIQUENOT NULL``的索引时会被使用。`
`ref`	对于前面表的结果集匹配查询的所有行，当连接使用索引key时，或者索引不是`PRIMARYKEY``和``UNIQUE``，`则使用该类型。如果使用索引匹配少量行时，是不错的连接类型。
`ref_or_null`	`连接类型类似``ref``，只是搜索的行中包含``NULL``值``MySQL``做了额外的查找。`
`fulltext`	使用全文索引时出现。
`index_merge`	使用了索引合并优化。(未成功)
`unique_subquery`	该类型将ref替换成以下子查询的格式： valueIN (SELECTprimary_key
`index_subquery`	与 `unique_subquery``类似，但是将主键改为非唯一索引：` valueIN (SELECTkey_columnFROMsingle_table WHERE some_expr)
`range`	使用索引检索给定范围内的行。
`index`	`该连接类型与``ALL``相同，除了扫描索引树。如果查询的字段都在索引列中，则使用``index``类型，否则为``ALL``类型。`
`ALL`	对于前面表的结果集中，进行了全表扫描。最差的一种类型，应考虑查询优化了！

查询类型性能由优到差：

system > const > eq_ref > ref > fulltext > ref_or_null > index_merge > unique_subquery > index_subquery > range > index > ALL

#system

mysql> explain select id from(select id from tabname where id=1) as a;
+----+-------------+------------+--------+---------------+---------+---------+------+------+-------------+
| id | select_type | table      | type   | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+------------+--------+---------------+---------+---------+------+------+-------------+
|  1 | PRIMARY     | <derived2> | system | NULL          | NULL    | NULL    | NULL |    1 |             |
|  2 | DERIVED     | tabname    | const  | PRIMARY       | PRIMARY | 4       |      |    1 | Using index |
+----+-------------+------------+--------+---------------+---------+---------+------+------+-------------+

#const

mysql> explain select * from tabname as a,tabname as b where a.id=b.id and a.id=1;
mysql> explain select * from tabname where id=1;
+----+-------------+---------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table   | type  | possible_keys | key     | key_len | ref   | rows | Extra |
+----+-------------+---------+-------+---------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | tabname | const | PRIMARY       | PRIMARY | 4       | const |    1 |       |
+----+-------------+---------+-------+---------------+---------+---------+-------+------+-------+

#eq_ref(engine=myisam)

mysql> explain select * from tabname2 as a,tabname2 as b where a.id=b.id;
+----+-------------+-------+--------+---------------+---------+---------+-----------+------+-------+
| id | select_type | table | type   | possible_keys | key     | key_len | ref       | rows | Extra |
+----+-------------+-------+--------+---------------+---------+---------+-----------+------+-------+
|  1 | SIMPLE      | a     | ALL    | PRIMARY       | NULL    | NULL    | NULL      |    3 |       |
|  1 | SIMPLE      | b     | eq_ref | PRIMARY       | PRIMARY | 4       | test.a.id |    1 |       |
+----+-------------+-------+--------+---------------+---------+---------+-----------+------+-------+

#ref

mysql> explain select * from tabname as a,tabname as b where a.tid=b.tid and a.tid=2;
mysql> explain select * from tabname where tid=2;
+----+-------------+---------+------+---------------+------+---------+-------+------+-------------+
| id | select_type | table   | type | possible_keys | key  | key_len | ref   | rows | Extra       |
+----+-------------+---------+------+---------------+------+---------+-------+------+-------------+
|  1 | SIMPLE      | tabname | ref  | tid           | tid  | 5       | const |    1 | Using where |
+----+-------------+---------+------+---------------+------+---------+-------+------+-------------+

#ref_or_null

mysql> explain select id,tid from tabname where tid=2 or tid is null;
+----+-------------+---------+-------------+---------------+------+---------+-------+------+--------------------------+
| id | select_type | table   | type        | possible_keys | key  | key_len | ref   | rows | Extra                    |
+----+-------------+---------+-------------+---------------+------+---------+-------+------+--------------------------+
|  1 | SIMPLE      | tabname | ref_or_null | tid           | tid  | 5       | const |    2 | Using where; Using index |
+----+-------------+---------+-------------+---------------+------+---------+-------+------+--------------------------+

#fulltext

mysql> alter table tabname2 add fulltext(name);
mysql> explain select * from tabname2 where match(name) against('love');
+----+-------------+----------+----------+---------------+------+---------+------+------+-------------+
| id | select_type | table    | type     | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+----------+----------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | tabname2 | fulltext | name          | name | 0       |      |    1 | Using where |
+----+-------------+----------+----------+---------------+------+---------+------+------+-------------+

#index_merge(未成功)

mysql> explain select * from tabname where tid>1 or indate<now();
mysql> explain select * from tabname where (tid>1 or indate>now()) AND name<'kk';

#unique_subquery

mysql> explain select * from tabname where tid in(select id from tabname);
mysql> explain select * from tabname where id in(select id from tabname);
+----+--------------------+---------+-----------------+---------------+---------+---------+------+------+-------------+
| id | select_type        | table   | type            | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+--------------------+---------+-----------------+---------------+---------+---------+------+------+-------------+
|  1 | PRIMARY            | tabname | ALL             | NULL          | NULL    | NULL    | NULL |    3 | Using where |
|  2 | DEPENDENT SUBQUERY | tabname | unique_subquery | PRIMARY       | PRIMARY | 4       | func |    1 | Using index |
+----+--------------------+---------+-----------------+---------------+---------+---------+------+------+-------------+

#index_subquery

mysql> explain select * from tabname where tid in(select tid from tabname);
mysql> explain select * from tabname where id in(select tid from tabname);
+----+--------------------+---------+----------------+---------------+------+---------+------+------+--------------------------+
| id | select_type        | table   | type           | possible_keys | key  | key_len | ref  | rows | Extra                    |
+----+--------------------+---------+----------------+---------------+------+---------+------+------+--------------------------+
|  1 | PRIMARY            | tabname | ALL            | NULL          | NULL | NULL    | NULL |    3 | Using where              |
|  2 | DEPENDENT SUBQUERY | tabname | index_subquery | tid           | tid  | 5       | func |    1 | Using index; Using where |
+----+--------------------+---------+----------------+---------------+------+---------+------+------+--------------------------+

#range

mysql> explain select * from tabname where tid between 1 and 2;
mysql> explain select * from tabname where id>1;
+----+-------------+---------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table   | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+---------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | tabname | range | PRIMARY       | PRIMARY | 4       | NULL |    2 | Using where |
+----+-------------+---------+-------+---------------+---------+---------+------+------+-------------+

#index

mysql> explain select id,tid from tabname;
mysql> explain select tid from tabname;
+----+-------------+---------+-------+---------------+------+---------+------+------+-------------+
| id | select_type | table   | type  | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+---------+-------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | tabname | index | NULL          | tid  | 5       | NULL |    3 | Using index |
+----+-------------+---------+-------+---------------+------+---------+------+------+-------------+

#ALL

mysql> explain select * from tabname where tid<>2;
mysql> explain select * from tabname;
+----+-------------+---------+------+---------------+------+---------+------+------+-------+
| id | select_type | table   | type | possible_keys | key  | key_len | ref  | rows | Extra |
+----+-------------+---------+------+---------------+------+---------+------+------+-------+
|  1 | SIMPLE      | tabname | ALL  | NULL          | NULL | NULL    | NULL |    3 |       |
+----+-------------+---------+------+---------------+------+---------+------+------+-------+

【Extra】

该列输出关MySQL如何解决查询的额外信息。（下面列出部分常见的）

Extra	Meaning
`usingwhere`	使用过滤条件
`usingindex`	`从索引树中查找所有列`
`usingtemporary`	`使用临时表存储结果集，在使用``groupby``和``orderby``发生`
`selecttables optimized away`	没有groupby情况下使用min(),max(),或者count(*)
`usingfilesort`	`有排序`
`notexists`	在leftjoin中匹配一行之后将不再继续查询查询
`distinct`	查找到第一个匹配的行之后，MySQL则会停止对当前行的搜索
`impossiblewhere`	where子句总数失败的查询
`impossiblehaving`	`having``子句总数失败的查询`
`usingjoin buffer`	使用连接缓存
`Usingindex for group-by`	`与``Usingindex``类似，在使用``group-by``时可从索引中找到字段`

#using where

mysql> explain select * from tabname where id>2;
mysql> explain select * from tabname where tid=2;
+----+-------------+---------+------+---------------+------+---------+-------+------+-------------+
| id | select_type | table   | type | possible_keys | key  | key_len | ref   | rows | Extra       |
+----+-------------+---------+------+---------------+------+---------+-------+------+-------------+
|  1 | SIMPLE      | tabname | ref  | tid           | tid  | 5       | const |    1 | Using where |
+----+-------------+---------+------+---------------+------+---------+-------+------+-------------+

#using index

mysql> explain select tid from tabname;
+----+-------------+---------+-------+---------------+------+---------+------+------+-------------+
| id | select_type | table   | type  | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+---------+-------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | tabname | index | NULL          | tid  | 5       | NULL |    3 | Using index |
+----+-------------+---------+-------+---------------+------+---------+------+------+-------------+

#using temporary

mysql> explain select distinct name from tabname;
+----+-------------+---------+------+---------------+------+---------+------+------+-----------------+
| id | select_type | table   | type | possible_keys | key  | key_len | ref  | rows | Extra           |
+----+-------------+---------+------+---------------+------+---------+------+------+-----------------+
|  1 | SIMPLE      | tabname | ALL  | NULL          | NULL | NULL    | NULL |    3 | Using temporary |
+----+-------------+---------+------+---------------+------+---------+------+------+-----------------+

#select tables optimized away

mysql> explain select max(tid) from tabname;
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra                        |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
|  1 | SIMPLE      | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | Select tables optimized away |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+

#using filesort

mysql> explain select id,name from tabname group by id,name;
mysql> explain select * from tabname order by name;
+----+-------------+---------+------+---------------+------+---------+------+------+----------------+
| id | select_type | table   | type | possible_keys | key  | key_len | ref  | rows | Extra          |
+----+-------------+---------+------+---------------+------+---------+------+------+----------------+
|  1 | SIMPLE      | tabname | ALL  | NULL          | NULL | NULL    | NULL |    3 | Using filesort |
+----+-------------+---------+------+---------------+------+---------+------+------+----------------+

#not exists

mysql> explain select * from tabname a left join tabname b on a.id=b.id where b.id is null;
+----+-------------+-------+--------+---------------+---------+---------+-----------+------+-------------------------+
| id | select_type | table | type   | possible_keys | key     | key_len | ref       | rows | Extra                   |
+----+-------------+-------+--------+---------------+---------+---------+-----------+------+-------------------------+
|  1 | SIMPLE      | a     | ALL    | NULL          | NULL    | NULL    | NULL      |    3 |                         |
|  1 | SIMPLE      | b     | eq_ref | PRIMARY       | PRIMARY | 4       | test.a.id |    1 | Using where; Not exists |
+----+-------------+-------+--------+---------------+---------+---------+-----------+------+-------------------------+

#distinct

mysql> explain select distinct a.id from tabname a left join tabname b on a.id=b.id;
+----+-------------+-------+--------+---------------+---------+---------+-----------+------+------------------------------+
| id | select_type | table | type   | possible_keys | key     | key_len | ref       | rows | Extra                        |
+----+-------------+-------+--------+---------------+---------+---------+-----------+------+------------------------------+
|  1 | SIMPLE      | a     | index  | NULL          | tid     | 5       | NULL      |    3 | Using index; Using temporary |
|  1 | SIMPLE      | b     | eq_ref | PRIMARY       | PRIMARY | 4       | test.a.id |    1 | Using index; Distinct        |
+----+-------------+-------+--------+---------------+---------+---------+-----------+------+------------------------------+

#impossible where

mysql> explain select * from tabname where 1=2;
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra            |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+
|  1 | SIMPLE      | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | Impossible WHERE |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+

#impossible having

mysql> explain select id,count(*) from tabname group by id having 1=2;
mysql> explain select count(*) from tabname having 1=2;
+----+-------------+-------+------+---------------+------+---------+------+------+-------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra             |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------------+
|  1 | SIMPLE      | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | Impossible HAVING |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------------+

#usingjoin buffer

#Using index for group-by

=====================================================================================

现在使用 EXTENDED 情况：

语法：

EXPLAIN EXTENDED <select statement>;

不使用 extended 和使用extended 的分析情况：

mysql> explain select tid from tabname;
+----+-------------+---------+-------+---------------+------+---------+------+------+-------------+
| id | select_type | table   | type  | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+---------+-------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | tabname | index | NULL          | tid  | 5       | NULL |    3 | Using index |
+----+-------------+---------+-------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> explain extended select tid from tabname;
+----+-------------+---------+-------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table   | type  | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+---------+-------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | tabname | index | NULL          | tid  | 5       | NULL |    3 |   100.00 | Using index |
+----+-------------+---------+-------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

可以看到，使用

extended 时，输出的最下面多了 1 条警告。此时可以用

show warnings 来查看：

mysql> show warnings \G;
*************************** 1. row ***************************
  Level: Note
   Code: 1003
Message: select `test`.`tabname`.`tid` AS `tid` from `test`.`tabname`
1 row in set (0.00 sec)

ERROR:
No query specified

show warnings 显示了优化器中是怎么规范表和字段名的，在通过重写和优化规则之后的 select 语句是什么样子。

更多参考：

EXPLAIN Output Format

EXPLAIN EXTENDED Output Format

标签：EXPLAIN,汇总,tabname,key,MySQL,NULL,type,id,select
From： https://blog.51cto.com/hzc2012/6024084

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