MySQL 8.4 Release Notes
In addition to storing metadata about database objects in the data
dictionary, MySQL stores it in serialized form. This data is
referred to as serialized dictionary information (SDI).
InnoDB stores SDI data within its tablespace
files. NDBCLUSTER stores SDI data in
the NDB dictionary. Other storage engines store SDI data in
.sdi files that are created for a given table
in the table's database directory. SDI data is generated in a
compact JSON format.
Serialized dictionary information (SDI) is present in all
InnoDB tablespace files except for temporary
tablespace and undo tablespace files. SDI records in an
InnoDB tablespace file only describe table and
tablespace objects contained within the tablespace.
SDI data is updated by DDL operations on a table or
CHECK TABLE FOR
UPGRADE. SDI data is not updated when the MySQL server
is upgraded to a new release or version.
The presence of SDI data provides metadata redundancy. For
example, if the data dictionary becomes unavailable, object
metadata can be extracted directly from InnoDB
tablespace files using the ibd2sdi tool.
For InnoDB, an SDI record requires a single
index page, which is 16KB in size by default. However, SDI data is
compressed to reduce the storage footprint.
For partitioned InnoDB tables comprised of
multiple tablespaces, SDI data is stored in the tablespace file of
the first partition.
The MySQL server uses an internal API that is accessed during DDL operations to create and maintain SDI records.
The IMPORT TABLE statement imports
MyISAM tables based on information contained in
.sdi files. For more information, see
Section 15.2.6, “IMPORT TABLE Statement”.