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2020-03-12-invisible-index.md

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Proposal: Invisible index

Abstract

MySQL supports invisible indexes; that is, indexes that are not used by the optimizer.

This is a useful feature for dropping an index in a safe way. 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 turn out to be required. Dropping and re-adding an index can be expensive for a large table, whereas making it invisible and visible are fast, in-place operations.

Support the option of VISIBLE | INVISIBLE.

CREATE [...] INDEX index_name
    [index_type]
    ON tbl_name (key_part,...)
    [index_option]

index_option:
    {VISIBLE | INVISIBLE}

Also, the following behaviors need to be supported as well:

  • Display information about invisible indexes in the output of INFORMATION_SCHEMA.STATISTICS or SHOW INDEX.
  • Index hints need to report errors when used in invisible indexes.
  • The primary key cannot be set to an invisible index.

Background

Index has a great influence on the performance of the database. Whether there is an index or whether the optimizer chooses the right index determines the performance of reading and writing the database to a great extent. In some cases, we would like to know the impact of deleting an index on the read-write performance of the database. At present, our method is to tell the optimizer to ignore the index through Index Hint. Although this method can achieve the goal, it is not practical to modify all SQL statements.

Proposal

Adding an option (visible or not) to the index. If it is not visible, it's called Invisible Index. Invisible indexes cannot be used by the optimizer (with the use_invisible_indexes switch on), but the index is maintained during DML operations. For a query statement, invisible indexes have the same effect as ignoring the index through Index Hint.

The option INVISIBLE of an index can be changed by using the following DDL statement:

ALTER TABLE table_name ALTER INDEX index_name { INVISIBLE | VISIBLE };

Or by setting option INVISIBLE when creating an index:

CREATE INDEX index_name ON table_name(key) [ INVISIBLE | VISIBLE ];

INVISIBLE, VISIBLE is as part of the index option, they can also be set when creating a table.

In order to know whether an index is invisible, it can be read from the INFORMATION_SCHEMA.STATISTICS table or through the SHOW INDEX command.

In addition, add a new flag use_invisible_indexes in system variable optimizer_switch, which determine whether the option INVISIBLE takes effect. If use_invite_indexes is on, the optimizer can still use invisible index.

A table with no explicit primary key may still have an effective implicit primary key if it has any UNIQUE indexes on NOT NULL columns. In this case, the first such index places the same constraint on table rows as an explicit primary key and that index cannot be made invisible.

Rationale

Another solution for implement invisible indexes is: Indicate invisibility by DDL state WriteOnly. This solution has the following problems:

  • The logic of schema change needs to be changed, which is relatively complicated to implement.
  • Cannot distinguish between indexes that are currently in WriteOnly and invisible indexes.
  • Handling the switch is troublesome.

Compatibility and Migration Plan

This the a new feature and it's absolutely compatible with old TiDB versions, also, it does not impact any data migration. The syntax and functions are basically compatible with MySQL expect:

When use invisible index in `SQL Hint`, and set `use_invisible_indexes = false`, MySQL allow use the invisible index.
But in TiDB, It's **not allowed** and an `Unresolved name` error will be thrown.

Implementation

  • Add syntax support in parser
  • Add a new column IS_VISIBLE in information_schema.statistics
  • Add a new column VISIBLE in SHOW INDEX FROM table statement
  • Show invisible column information in SHOW CREATE TABLE statement
  • Add use_invisible_indexes in system variable @@optimizer_switch
  • Add new error message ERROR 3522 (HY000): A primary key index cannot be invisible
  • Ignore invisible index in optimizer and add unit tests

Testing Plan

  • Unit test
  • Learn from MySQL test cases related to invisible indexes

Open issues

pingcap#9246