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This article is to share with you about the functions of MySQL indexes. The editor thinks it is very practical, so share it with you as a reference and follow the editor to have a look.

I. introduction to the index

(1) the meaning and specificity of the index

(2) Classification of indexes

(3) the design principle of index.

Second, create an index

(1) create an index when creating a table

(2) create an index on an existing table

(3) Delete the index

I. introduction to the index

The index is used to quickly find rows that have a specific value in a column. Without using an index, MySQL must read the entire table from record 1 until the relevant rows are found. The larger the table, the more time it takes to query the data. If the query column in the table has an index, MySQL can quickly get to a location to search for data files without having to view all the data.

(1) the meaning and specificity of the index

Meaning: an index is a separate database structure stored on disk that contains reference pointers to all records in the data table. Used to quickly find rows that have a specific value in one or more columns.

Indexes are implemented in the storage engine, so the indexes of each storage engine are not necessarily the same, and each storage engine does not necessarily support all index types. Define the maximum number of indexes and the maximum index length for each table according to the storage engine. All storage engines support at least 16 indexes per table, with a total index length of at least 256 bytes. Most storage engines have higher limits.

There are two storage types of indexes in MySQL: BTREE and HASH, which are related to the table storage engine; MyISAM and InnoDB storage engines only support BTREE indexes; and MEMORY/HEAL storage engines can support HASH and BTREE indexes.

Advantages of the index:

1. By creating a unique index, you can ensure the uniqueness of each row of data in the database table.

two。 It can greatly speed up the query speed of data. (the main reason for creating an index)

3. In terms of achieving the referential integrity of the data, the connection between the table and the table can be accelerated.

4. When you use grouping and sorting clauses for data queries, you can also significantly reduce the time for grouping and sorting in the query.

Disadvantages of adding indexes:

1. It takes time to create and maintain the index, and the time spent increases as the amount of data increases.

two。 Indexes occupy disk space, in addition to data tables occupy data space, each index also takes up a certain amount of physical space. If there are a large number of indexes, the index file may reach the maximum file size faster than the data file.

3. When the data in the table is added, deleted and modified, the index should also be maintained dynamically, which reduces the maintenance speed of the data.

(2) Classification of indexes

1. General index and unique index (unique)

A normal index is a basic index type in MySQL that allows duplicate and null values to be inserted in the columns that define the index.

Unique index, the value of the index column must be unique, but null values are allowed. If it is a combined index, the combination of column values must be unique.

The primary key index is a special unique index, and null values are not allowed.

two。 Single-column index and combined index

Single column so you have an index that contains only a single column, and a table can have multiple single column indexes.

A combined index is an index created on a combination of fields in a table, and the index is used only if the left fields of these fields are used in the query criteria.

Follow the leftmost prefix collection when using a combined index.

3. Full text Index (fulltext)

The full-text index type is FULLTEXT and supports full-text lookup of values on columns that define the index, allowing duplicate and null values to be inserted in these index columns, and full-text indexes can be created on columns of type char, varchar, or text. Full-text indexing is supported only by the MyISAM storage engine in MySQL.

4. Spatial index (spatial)

Spatial index is an index established on the fields of spatial data types. There are four spatial data types in MySQL, namely geometry,point,linestring and polygon. MySQL is extended with the spatial keyword to make it possible to create spatial indexes in a syntax similar to that used to create regular indexes. The column that creates the spatial index must be declared as not null, and the spatial index can only be created in tables where the storage engine is MySQL.

(3) the design principle of index.

Unreasonable index design or lack of indexes can hinder the performance of databases and applications. Efficient indexes are important for good performance, and the following guidelines should be considered when designing indexes:

1. The more indexes, the better.

two。 Avoid too many indexes on frequently updated tables and index as few columns as possible.

3. It is best not to use indexes for tables with a small amount of data.

4. Index on columns with more different values that are often used in conditional expressions, and do not index columns with few different values.

5. When uniqueness is a characteristic of the data itself, specify a unique index.

6. Index on columns that are frequently sorted or grouped (for group by or order by operations), and if there are more than one column to be sorted, you can build a combined index on those columns.

Second, create an index

Syntax format:

Create table table_name [col_name date_type] [unique | fulltext | spatial] [index | key] [index_name] (col_name [length]) [asc | desc]

Unique,fulltext and spatial are optional parameters, representing unique index, full-text index and spatial index, respectively.

Index and key are synonyms for the same purpose and are used to specify the creation of an index.

Col_name is the field column that needs to be indexed and must be selected from multiple columns defined in the data table.

Index_name specifies the name of the index, which is an optional parameter. If not, MySQL defaults to the index value of col_name.

Length is an optional parameter that indicates the length of the index. Only fields of type string can specify the length of the index.

Asc or desc specifies the index value store for ascending or descending order.

(1) create an index when creating a table

① creates a normal index

Ordinary index is the most basic type of index, there are no restrictions such as uniqueness, its function is to speed up the access to data.

[example 1] create a general index on the year_publication field in the book table. The SQL statement is as follows:

Mysql > create table book-> (- > bookid int not null,-> bookname varchar (255not null),-> authors varchar (255not null),-> info varchar (255null),-> comment varchar (255null),-> year_publication year not null,-> index (year_publication)->) Query OK, 0 rows affected (0.21 sec) mysql > show create table book\ Graph * 1. Row * * Table: bookCreate Table: CREATE TABLE `book` (`bookid` int (11) NOT NULL, `bookname` varchar (255) NOT NULL, `authors`varchar (255) NOT NULL, `info` varchar (255) DEFAULT NULL `comment` varchar (255) DEFAULT NULL, `year_ publication` year (4) NOT NULL KEY `year_ publication` (`year_ publication`) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci1 row in set (0.06 sec) mysql > explain select * from book where year_publication=1990\ gateway * 1. Row * * id: 1 select_type: SIMPLE table: book partitions : NULL type: refpossible_keys: year_publication key: year_publication key_len: 1 ref: const rows: 1 filtered: 100.00 Extra: NULL1 row in set 1 warning (0.00 sec)

The lines of the output of the explan statement are interpreted as follows:

The select_type line specifies the type of select query to be used, where the value is simple, which represents a simple select, without using union or subqueries. Other possible values include primary, union, subquery, etc.

The table row specifies the names of the data tables read by the database, arranged in the order in which they were read.

The type row specifies the association between this database table and other database tables. Possible values are system, const, eq_ref, ref, range, index, and all.

Possible_keys line. The indexes that can be selected by MySQL when searching data records are given.

The key row is the index that MySQL actually chooses.

The key_len line gives the length of the index in bytes, and the smaller the key_len value, the faster it is.

The ref row gives the name of the data column in another data table in the associated relationship.

The rows row is the number of data rows that MySQL is expected to read from this data table when executing the query.

The extra line provides information about the associated operation.

As you can see, the values of both possible_key and key are year_publication, and the index is used in the query.

② creates a unique index

The main reason for creating unique indexes is to reduce the execution time of query index column operations, especially for larger data tables. It is similar to the previous normal index, except that the value of the index column must be unique, but null values are allowed. If it is a combined index, the combination of column values must be unique.

[example 2] create a table T1 and use the unique keyword to create a unique index on the id field in the table.

Mysql > create table T1-> (- > id int not null->, name char (30) not null,-> unique index uniqidx (id)->) Query OK, 0 rows affected (0.27 sec) mysql > show create table T1\ Graph * 1. Row * * Table: t1Create Table: CREATE TABLE `t1` (`id` int (11) NOT NULL, `name` char (30) NOT NULL UNIQUE KEY `uniqidx` (`id`) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci1 row in set (0.06 sec)

③ creates a single-column index

A single-column index is an index created on a field in a data table, and multiple single-column indexes can be created in a table.

[example 3] create a table T2 and create a single-column index on the name field in the table.

Mysql > create table T2-> (- > id int not null,-> name char (50) null,-> index singleidx (name (20))->) Query OK, 0 rows affected (0.06 sec) mysql > show create table T2\ row * * Table: t2Create Table: CREATE TABLE `t2` (`id` int (11) NOT NULL, `name` char (50) DEFAULT NULL KEY `singleidx` (`name` (20)) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci1 row in set (0.01sec)

As you can see from the result, a single-column index named SingleIdx has been successfully established on the id field with an index length of 20.

④ creates a composite index

A combined index is to create an index on multiple fields.

[example 4] create table T3 and create a combined index on the id, name and age fields in the table. The SQL statement is as follows:

Mysql > create table T3-> (- > id int not null,-> name char (30) not null,-> age int not null,-> info varchar (255),-> index mulitiidx (id,name,age)->) Query OK, 0 rows affected (0.07 sec) mysql > show create table T3\ show create table * 1. Row * * Table: t3Create Table: CREATE TABLE `t3` (`id` int (11) NOT NULL, `name` char (30) NOT NULL, `age`int (11) NOT NULL, `info` varchar (255) DEFAULT NULL, KEY `mulitiidx` (`id`, `name`) `age`) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci1 row in set (0.06 sec) mysql > explain select * from T3 where id = 1 and name = 'joe'\ gateway * 1. Row * * id: 1 select_type: SIMPLE table: T3 partitions: NULL type: refpossible_keys: mulitiidx key: mulitiidx key_len: 124 ref: const Const rows: 1 filtered: 100.00 Extra: Using index condition1 row in set, 1 warning (0.06 sec)

The combined index causes the effect of several indexes, but the index can not be used to query any field, but it follows the "leftmost prefix": the leftmost column set in the index is used to match the row. such a column set is called the leftmost prefix.

For example, here is an index consisting of three id,name,age fields. The index row is extensive in the order of id/nam/age, and the index can search for a combination of fields: (id,name,age), (id, name), or id. If the column does not form the leftmost prefix of the index, MySQL cannot use a local index, such as a combination of (age) or (name,age). When querying the id and name fields, the index of multiidx is used. If the query (name,age) is combined or queries the name and age fields separately, the index is null.

⑤ creates a full-text index

Fulltext full-text index can be used for full-text search. Only the MyISAM storage engine supports fulltext indexes, and indexes are created only for char, varchar, and text columns. The index is always on the entire column and does not support local (prefix) indexes.

[example 5] create table T4 and create a full-text index on the info field in the table. The SQL statement is as follows:

Mysql > create table T4-> (- > id int not null,-> name char (30) not null,-> age int not null,-> info varchar (255),-> fulltext index fulltxtidx (info)->) engine=MyISAM Query OK, 0 rows affected (0.08 sec) mysql > show create table T4\ Graph * 1. Row * * Table: t4Create Table: CREATE TABLE `t4` (`id` int (11) NOT NULL, `name` char (30) NOT NULL, `age` int (11) NOT NULL, `info` varchar (255) DEFAULT NULL FULLTEXT KEY `fulltxtidx` (`info`) ENGINE=MyISAM DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci1 row in set (0.06 sec)

Full-text indexing is very suitable for large data sets, but it is less useful for small data sets.

6. Create a spatial index

A spatial index must be created in a table of type MyISAM, and fields of type space must be non-empty.

[example 6] create table T5 and create a spatial index on the field with spatial type geometry. The SQL statement is as follows:

Mysql > create table T5-> (g geometry not null,spatial index spatidx (g)) ENGINE=MyISAM Query OK, 0 rows affected, 1 warning (0.07 sec) mysql > show create table t5\ row * * Table: t5Create Table: CREATE TABLE `t5` (`g` geometry NOT NULL, SPATIAL KEY `spatidx` (`g`)) ENGINE=MyISAM DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci1 row in set (0.05sec)

As you can see, a spatial index named spatIdx is created on the g field of the T5 table. A non-empty constraint that specifies the field value of the space type when created, and the storage engine of the table is MyISAM.

(2) create an index on an existing table

To create an index in an existing table, you can use an alter table statement or a create index statement.

1. Create an index using the alter table statement

Basic syntax:

Alter table table_name add [unique | fulltext | spatial] [index | key] [index_name] (col_ name [length],...) [asc | dec]

[example 7] create a normal index named BkNameIdx on the bookname field in the book table.

Before adding an index, the show index statement looks at the index created in the specified table:

Mysql > show index from book\ gateway * 1. Row * * Table: book Non_unique: 1 Key_name: year_publication Seq_in_index: 1 Column_name: year_publication Collation: a Cardinality: 0 Sub_part: NULL Packed: NULL Null: Index_type: BTREE Comment:Index_comment: Visible: YES Expression: NULL1 row in set (0.10 sec)

Among them, the meaning of each main parameter is

Table represents the table on which the index is created.

Non_unique indicates that the index is not unique, 1 is not unique, and 0 is unique.

Key_name represents the name of the index.

Seq_in_index represents the position of the field in the index, the value of the single-column index is 1, and the combined index is the order of each field in the index definition.

Column_name represents the column field that defines the index.

Sub_part represents the length of the index.

Null indicates whether the field can be null.

Index_type represents the index type.

You can see that an index already exists in the book table, that is, the year_publication index, which is a non-unique index. Use alter table to add an index on the bookname field, as shown in the SQL statement:

Mysql > alter table book add index bknameidx (bookname (30)) Query OK 0 rows affected (0.09 sec) Records: 0 Duplicates: 0 Warnings: 0mysql > show index from book\ gateway * 1. Row * * Table: book Non_unique: 1 Key_name: year_publication Seq_in_index: 1 Column_name: year_publication Collation: A Cardinality: 0 Sub_part: NULL Packed: NULL Null: Index_type: BTREE Comment:Index_comment: Visible: YES Expression: NULL** 2. Row * * Table: book Non_unique: 1 Key _ name: bknameidx Seq_in_index: 1 Column_name: bookname Collation: a Cardinality: 0 Sub_part: 30 Packed: NULL Null: Index_type: BTREE Comment:Index_comment: Visible: YES Expression: NULL2 rows in set (0.05 sec)

You can see that there are two indexes in the table, and the other is an index named bknameidx added by the alter table statement, which is a non-unique index with a length of 30.

[example 8] create a unique index named uniqididx on the bookid field of the book table. The SQL statement is as follows:

Mysql > alter table book add unique index uniqididx (bookid) Query OK 0 rows affected (0.17 sec) Records: 0 Duplicates: 0 Warnings: 0mysql > show index from book\ G1.. 2. Row * * Table: book Non_unique: 1 Key_name: bknameidx Seq_in_index: 1 Column_name: bookname Collation: a Cardinality: 0 Sub_part: 30 Packed: NULL Null: Index_type: BTREE Comment:Index_comment: Visible: YES Expression: NULL3 rows in set (0.01 sec)

As you can see, the attribute value of Non_unique is 0, which means that the index named Uniqididx is a unique index, and the unique index was created successfully.

[example 9] create a single-column index on the comment field of the book table. The SQL statement is as follows:

Mysql > alter table book add index bkcmtidx (comment (50)) Query OK 0 rows affected (0.11 sec) Records: 0 Duplicates: 0 Warnings: 0mysql > show index from book\ G1.. 2. 3. Row * * Table: book Non_unique: 1 Key_name: bkcmtidx Seq_in_index: 1 Column _ name: comment Collation: a Cardinality: 0 Sub_part: 50 Packed: NULL Null: YES Index_type: BTREE Comment:Index_comment: Visible: YES Expression: NULL4 rows in set (0.01 sec)

As you can see, after the statement is executed, an index named bkcmtidx is established on the comment field of the book table, with a length of 50. When querying, only the first 50 characters need to be retrieved.

[example 10] create a combined index on the authors and info fields of the book table. The SQL statement is as follows:

Mysql > alter table book add index bkauandinfoidx (authors (30), info (50)) Query OK 0 rows affected (0.09 sec) Records: 0 Duplicates: 0 Warnings: 0mysql > show index from book\ G1. 2. 3. 4. Row * * Table: book Non_unique: 1 Key_name: bkauandinfoidx Seq_in_ Index: 1 Column_name: authors Collation: a Cardinality: 0 Sub_part: 30 Packed: NULL Null: Index_type: BTREE Comment:Index_comment: Visible: NULL** 6. Row * * Table : book Non_unique: 1 Key_name: bkauandinfoidx Seq_in_index: 2 Column_name: info Collation: a Cardinality: 0 Sub_part: 50 Packed: NULL Null: YES Index_type: BTREE Comment:Index_comment: Visible: YES Expression: NULL6 rows in set (0.06 sec)

You can see that the index named bkauandinfoidx consists of two fields, the length of the authors field is 30, and the ordinal in the combined index is 1. This field does not allow a null null;info field with a length of 50, and an ordinal of 2 in the combined index, which can be a null null.

[example 11] create table T6, and use alter table to create a full-text index on table T6. The SQL statement is as follows:

Mysql > create table T6-> (- > id int not null,-> info char (255)->) ENGINE=MyISAM;Query OK, 0 rows affected (0.07 sec) mysql > alter table T6 add fulltext index infofiidx (info) Query OK 0 rows affected (0.13 sec) Records: 0 Duplicates: 0 Warnings: 0mysql > show index from T6\ Graph * 1. Row * * Table: T6 Non_unique: 1 Key_name: infofiidx Seq_in_index: 1 Column_name: info Collation: NULL Cardinality: NULL Sub_part: NULL Packed: NULL Null: YES Index_type: FULLTEXT Comment:Index_comment: Visible: YES Expression: NULL1 row in set (0.05sec)

As you can see, an index named infoftidx has been created in the T6 table, which is created on the info field, of type fulltext, and allows null values.

[example 12] create a spatial index named spatidx on the space type field g of table t7 ~ T7. The SQL statement is as follows:

Mysql > create table T7 (g geometry not null) ENGINE=MyISAM;Query OK, 0 rows affected (0.07 sec) mysql > alter table T7 add spatial index spatidx (g) Query OK, 0 rows affected 1 warning (0.06 sec) Records: 0 Duplicates: 0 Warnings: 1mysql > show index from T7\ Graph * 1. Row * * Table: T7 Non_unique: 1 Key_name: spatidx Seq_in_index: 1 Column_name: G Collation: a Cardinality: NULL Sub_part: 32 Packed: NULL Null: Index_type: SPATIAL Comment:Index_comment: Visible: YES Expression: NULL1 row in set (0.01 sec)

As you can see, a spatial index named spatidx is created on the g field of the T7 table.

two。 Create an index using create index

The create index statement can add an index to an existing table, and create index is mapped to an alter table statement in MySQL. The basic syntax is as follows:

Create [unique | fulltext | spatial] index index_nameon table_name (col_ name [length],...) [asc | desc]

You can see that the syntax of the create index statement is basically the same as that of the alter index statement, except that the keywords are different. Use the same table book, assuming that there are no index values in the table. Create the book table statement as follows:

Create table book (bookid int not null,bookname varchar (255) not null,authors varchar (255) not null,info varchar (255) null,comment varchar (255) null,year_publication year not null)

[example 13] create a general index named BkNameIdx on the bookname field of the book table. The SQL statement is as follows:

Mysql > create index BkNameOdx on book (bookname); Query OK, 0 rows affected (0.10 sec) Records: 0 Duplicates: 0 Warnings: 0

[example 14] create a unique index named UniqidIdx on the bookid field of the book table. The SQL statement is as follows:

Mysql > create unique index UniqiiIdx on book (bookid); Query OK, 0 rows affected (0.05 sec) Records: 0 Duplicates: 0 Warnings: 0

[example 15] create a single-column index on the comment field of the book table. The SQL statement is as follows:

Mysql > create index BkcmtIdx on book (bookid); Query OK, 0 rows affected (0.08 sec) Records: 0 Duplicates: 0 Warnings: 0

[example 16] create a combined index on the authors and info fields of the book table. The SQL statement is as follows:

Mysql > create index BkAuAndInfoIdx on book (authors (20), info (50)); Query OK, 0 rows affected (0.09 sec) Records: 0 Duplicates: 0 Warnings: 0

[example 17] delete table T6, re-establish table T6, use create index statements in T6 table, and create a full-text index on the info field of type char.

Mysql > drop table T6 politics query OK, 0 rows affected (0.02 sec) mysql > create table T6-> (- > id int not null,-> info char (255)->) ENGINE=MyISAM;Query OK, 0 rows affected (0.06 sec) mysql > create fulltext index infoftidx on T6 (info); Query OK, 0 rows affected (0.07 sec) Records: 0 Duplicates: 0 Warnings: 0

[example 18] delete table T7, recreate table T7, use the create index statement in table T7, and create a spatial index named spatIdx on the spatial data type field g.

Mysql > drop table t7 sec query OK, 0 rows affected (0.06 sec) mysql > create table T7 (g geometry not null) ENGINE=MyISAM;Query OK, 0 rows affected (0.06 sec) mysql > create spatial index spatIdx on T7 (g); Query OK, 0 rows affected, 1 warning (0.07 sec) Records: 0 Duplicates: 0 Warnings: 1 (3) Delete Index

Delete indexes in MySQL use alter table or drop index statements, both of which achieve the same function, and drop index statements are internally mapped to an alter table statement.

1. Delete an index using alter table

The basic syntax format of the alter table delete index:

Alter table table_name drop index index_name

[example 1] Delete the unique index named UniqidIdx in the book table.

Mysql > show create table book\ Graph * 1. Row * * Table: bookCreate Table: CREATE TABLE `book` (`bookid` int (11) NOT NULL, `bookname` varchar (255) NOT NULL, `authors`varchar (255) NOT NULL, `info` varchar (255) DEFAULT NULL, `comment`varchar (255) DEFAULT NULL, `year_ publication` year (4) NOT NULL UNIQUE KEY `UniqiIdx` (`bookid`), KEY `BkNameOdx` (`bookname`), KEY `BkcmtIdx` (`bookid`), KEY `BkAuAndInfoIdx` (`authors` (20), `info`) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci1 row in set (0.00 sec) mysql > alter table book drop index UniqiIdx Query OK, 0 rows affected (0.19 sec) Records: 0 Duplicates: 0 Warnings: 0mysql > show create table book\ Graph * 1. Row * * Table: bookCreate Table: CREATE TABLE `book` (`bookid` int (11) NOT NULL, `bookname` varchar (255) NOT NULL, `authors`varchar (255) NOT NULL, `info` varchar (255) DEFAULT NULL `comment` varchar (255) DEFAULT NULL, `year_ publication` year (4) NOT NULL, KEY `BkNameOdx` (`bookname`), KEY `BkcmtIdx` (`bookid`), KEY `BkAuAndInfoIdx` (`authors` (20), `info` (50)) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci1 row in set (0.00 sec)

You can see that there is no unique index named UniqidIdx in the book table, and the index was deleted successfully.

Note: a unique index that adds an auto_increment constraint field cannot be deleted.

two。 Use the drop index statement to delete the index

The drop index statement removes the basic syntax format of the index:

Drop index inde _ name on table_name

[example 2] Delete the composite index named BkAuAndInfoIdx in the book table. The SQL statement is as follows:

Mysql > drop index BkAuAndInfoIdx on book Query OK, 0 rows affected (0.08 sec) Records: 0 Duplicates: 0 Warnings: 0mysql > show create table book\ Graph * 1. Row * * Table: bookCreate Table: CREATE TABLE `book` (`bookid` int (11) NOT NULL, `bookname` varchar (255) NOT NULL, `authors`varchar (255) NOT NULL, `info` varchar (255) DEFAULT NULL `comment` varchar (255) DEFAULT NULL, `year_ publication` year (4) NOT NULL, KEY `BkNameOdx` (`bookname`), KEY `BkcmtIdx` (`bookid`) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci1 row in set (0.00 sec)

You can see that there is no composite index named BkAuAndInfoIdx in the book table, and the index was deleted successfully.

Note: when you delete a column in a table, if the column you want to delete is part of the index, that part is also deleted from the index. If all the columns that make up the index are deleted, the entire index will be deleted.

Thank you for reading! This is the end of this article on "what is the function of MySQL index?". I hope the above content can be of some help to you, so that you can learn more knowledge. if you think the article is good, you can share it for more people to see!

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