Error 3522 a primary key index cannot be invisible - Исправление ошибок и поиск оптимальных решений проблем

Summary: in this tutorial, you will learn about MySQL invisible index and the statements to manage the index visibility.

Introduction to MySQL invisible index

The invisible indexes allow you to mark indexes as unavailable for the query optimizer. MySQL maintains the invisible indexes and keeps them up to date when the data in the columns associated with the indexes changes.

By default, indexes are visible. To make them invisible, you have to explicitly declare its visibility at the time of creation, or by using the ALTER TABLE command. MySQL provides us with the VISIBLE and INVISIBLE keywords to maintain index visibility.

To create an invisible index, you the following statement:

CREATE INDEX index_name
ON table_name( c1, c2, ...) INVISIBLE;
Code language: SQL (Structured Query Language) (sql)

In this syntax:

First, you specify the name of the index after the CREATE INDEX clause.
Second, you list the table name and the column list which you want to add to the index. The INVISIBLE keyword indicates that the index that you are creating is invisible.

For example, the following statement creates an index on the extension column of the employees table in the sample database and marks it as an invisible index:

CREATE INDEX extension 
ON employees(extension) INVISIBLE;
Code language: SQL (Structured Query Language) (sql)

To change the visibility of existing indexes, you use the following statement:

ALTER TABLE table_name
ALTER INDEX index_name [VISIBLE | INVISIBLE];
Code language: SQL (Structured Query Language) (sql)

For example, to make the extension index visible, you use the following statement:

ALTER TABLE employees
ALTER INDEX extension VISIBLE; 
Code language: SQL (Structured Query Language) (sql)

You can find the indexes and their visibility by querying the statistics table in the information_schema database:

SELECT 
    index_name, 
    is_visible
FROM
    information_schema.statistics
WHERE
    table_schema = 'classicmodels'
        AND table_name = 'employees';




  


                       
  



Code language: SQL (Structured Query Language) (sql)

Here is the output:

In addition, you can use the SHOW INDEXES command to display all indexes of a table:

SHOW INDEXES FROM employees;
Code language: SQL (Structured Query Language) (sql)

As mentioned earlier, the query optimizer does not use invisible index so why do you use the invisible index in the first place? Practically speaking, invisible indexes have a number of applications. For example, you can make an index invisible to see if it has an impact to the performance and mark the index visible again if it does.

MySQL invisible index and primary key

The index on the primary key column cannot be invisible. If you try to do so, MySQL will issue an error.

In addition, an implicit primary key index also cannot be invisible. When you defines a UNIQUE index on a NOT NULL column of a table that does not have a primary key, MySQL implicitly understands that this column is the primary key column and does not allow you to make the index invisible.

Consider the following example.

First, create a new table with a UNIQUE index on a NOT NULL column:

CREATE TABLE discounts (
    discount_id INT NOT NULL,
    name VARCHAR(50) NOT NULL,
    valid_from DATE NOT NULL,
    valid_to DATE NOT NULL,
    amount DEC(5 , 2 ) NOT NULL DEFAULT 0,
    UNIQUE discount_id(discount_id)
);
Code language: SQL (Structured Query Language) (sql)

Second, try to make the discount_id index invisible:

ALTER TABLE discounts
ALTER INDEX discount_id INVISIBLE;
Code language: SQL (Structured Query Language) (sql)

MySQL issued the following error message:

Error Code: 3522. A primary key index cannot be invisible   
Code language: SQL (Structured Query Language) (sql)

MySQL invisible index system variables

To control visible indexes used by the query optimizer, MySQL uses the use_invisible_indexes flag of the optimizer_switch system variable. By default, the use_invisible_indexes is off:

SELECT @@optimizer_switch;
Code language: SQL (Structured Query Language) (sql)

In this tutorial, you have learned about the MySQL invisible index, how to create an invisible index, and how to change the visibility of an existing index.

Was this tutorial helpful?

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Overview

In this blog, we aim to discuss the new MySQL 8.0 feature: invisible indexes and how to manage index visibility.

MySQL 8.0 supports invisible indexes, that is, indexes not used by the optimizer. This feature applies to indexes other than primary keys (either explicit or implicit).

Starting from MySQL 8.0, MySQL has introduced a new feature: invisible indexes. These invisible indexes allow you to mark indexes as unavailable/invisible to the query optimizer. MySQL maintains the invisible indexes and keeps them up to date whenever the data in the columns associated with the indexes changes.

By default, indexes in MySQL are visible. To make them invisible, we have to explicitly declare their visibility at the time of creation or by using the ALTER TABLE command. This is an online DDL operation which is done in a non-locking manner. MySQL provides us with the VISIBLE and INVISIBLE keywords to maintain this index visibility.

Index visibility does not affect index maintenance. An index still continues to be updated according to the changes made to table rows, and a unique index still prevents the insertion of duplicates into a column, regardless of whether the index is visible or invisible.

How to Create Invisible Indexes

To create a new invisible index, you can issue the following statement:

CREATE INDEX index_name ON table_name (idx1) INVISIBLE;

In this syntax, the INVISIBLE keyword indicates that the index you are creating is hidden; if not specified, the index will be made VISIBLE by default.

If you wish to change the visibility of an existing index, you use the following statement:

ALTER TABLE table_name ALTER INDEX index_name INVISIBLE;

Once done, you can find the indexes and their visibility by querying the statistics table in the information_schema database:

SELECT index_name, is_visible FROM information_schema.statistics WHERE table_schema = DB_name AND table_name = table_name;

If you wish to change the invisible indexes back to being visible, this command can be used

ALTER TABLE table_name ALTER INDEX index_name VISIBLE;

To control visible indexes used by the query optimizer, MySQL uses the use_invisible_indexes flag of the optimizer_switch system variable. By default, the use_invisible_indexes is off:

SELECT @@optimizer_switch;

Benefits of Using this Feature

As mentioned earlier, the query optimizer does not use invisible indexes, so why do we use invisible indexes?

To answer that, invisible indexes make it possible to test the effect of removing an index on query performance without making a destructive change that must be undone should the index be required.

Dropping and re-adding an index can be expensive for a large table, adding extra overhead to your system, whereas making it invisible and visible are fast and in-place operations, which happen in online and non-locking ways.

If an index is made invisible, and it is actually needed or used by the optimizer, there are a few ways to notice the effect of its absence on queries for the table like below:

Errors occur for queries that include index hints that refer to the invisible index
Performance Schema data shows an increase in workload for affected queries
Queries have different EXPLAIN execution plans
Queries appear in the slow query log that did not appear there previously

If any such index is necessary, it can be easily changed back to VISIBLE.

MySQL invisible index limitations

Please note that the index on a primary key column cannot be made invisible. If you try to do so, MySQL will issue an error.

In addition, an implicit primary key index also cannot be invisible. When you define a UNIQUE index on a NOT NULL column of a table that does not have a primary key, MySQL implicitly understands that this column is the primary key column and will not allow you to make that index invisible.

For, e.g.:

Consider the following table definition:

CREATE TABLE test (a INT NOT NULL,b INT NOT NULL,UNIQUE b_idx (b)) ENGINE = InnoDB;

The definition shows there is no explicit primary key defined here, but the UNIQUE index on NOT NULL column b places the same constraint on rows as a primary key, and hence it cannot be made invisible:

ALTER TABLE test ALTER INDEX b_idx INVISIBLE;

ERROR 3522 (HY000): A primary key index cannot be invisible.

Now, let’s say we add an explicit primary key to the table:

ALTER TABLE test ADD PRIMARY KEY (a);

This direct primary key cannot be made invisible now. But the unique index on b no longer acts as an implicit primary key, so it can be made invisible now.

ALTER TABLE test ALTER INDEX a_idx INVISIBLE;

ERROR 3522 (HY000): A primary key index cannot be invisible.

ALTER TABLE test ALTER INDEX b INVISIBLE;

Query OK, 0 rows affected (0.03 sec)

Conclusion

This blog describes MySQL invisible indexes, how to create an invisible index, and how to change the visibility of an existing index. I hope you found this blog helpful!

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Invisible indexes is new feature added in MySQL 8.0. This feature can help us to mark an index as unavailable for use by optimizer. This means that index will still be maintained and kept up to date in metadata dictionary as data is modified. These marked indexes are not permitted to be used by optimizer even by INDEX hint.

Indexes are visible by default. To control index visibility for a new index or existing one VISIBLE OR INVISIBLE keywords are used.

for example: If we have following table with index j_idx

CREATE TABLE t1 ( 

i INT, 

j INT, 

k INT, 

INDEX i_idx (i) INVISIBLE 

                                 ) ENGINE = InnoDB; 

CREATE INDEX j_idx ON t1 (j) INVISIBLE; 

ALTER TABLE t1 ADD INDEX k_idx (k) INVISIBLE;

To change the visibility of existing index we can use following statements.

ALTER TABLE t1 ALTER INDEX i_idx INVISIBLE; 

ALTER TABLE t1 ALTER INDEX i_idx VISIBLE;

Information about index, if it is visible or invisible can be extracted from INFORMATION_SCHEMA.STATISTICS as shown below.

mysql> SELECT INDEX_NAME, IS_VISIBLE 
FROM INFORMATION_SCHEMA.STATISTICS 
WHERE TABLE_SCHEMA = 'db1' AND TABLE_NAME = 't1'; 

+----------------+---------------+ 
| INDEX_NAME     | IS_VISIBLE | 
+----------------+---------------+ 
| i_idx          | YES           | 
| j_idx          | NO            | 
| k_idx          | NO            | 
+----------------+---------------+

Invisible Indexes can help us possible to test the effect of removing an index for query performance, without dropping or recreating index which are expensive operations.

Primary Key Index cannot be made invisible , if you try to do that you will get error as shown below

mysql> ALTER TABLE t2 ALTER INDEX j_idx INVISIBLE; 

ERROR 3522 (HY000): A primary key index cannot be invisible.

I hope this will help you all, Don’t forget to like, share and comment

Thanks and Regards

Raja M Naveed

Database support hints and tips

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Summary: in this tutorial, you will learn about MySQL invisible index and the statements to manage index visibility.