DELETE operations are performed on a table,
often the values of indexed columns (particularly the values of
secondary keys) are not in sorted order, requiring substantial I/O
to bring secondary indexes up to date.
has a change buffer that
caches changes to secondary index entries when the relevant
page is not in the
buffer pool, thus avoiding
I/O operations by not reading in the page from the disk. The
buffered changes are merged when the page is loaded to the buffer
pool, and the updated page is later flushed to disk using the
normal mechanism. The
InnoDB main thread merges
buffered changes when the server is nearly idle, and during a
Because it can result in fewer disk reads and writes, this feature is most valuable for workloads that are I/O-bound, for example applications with a high volume of DML operations such as bulk inserts.
However, the change buffer occupies a part of the buffer pool, reducing the memory available to cache data pages. If the working set almost fits in the buffer pool, or if your tables have relatively few secondary indexes, it may be useful to disable change buffering. If the working set entirely fits in the buffer pool, change buffering does not impose any extra overhead, because it only applies to pages that are not in the buffer pool.
You can control the extent to which
performs change buffering with the system configuration parameter
innodb_change_buffering. You can
turn on and off buffering for inserts, delete operations (when
index records are initially marked for deletion) and purge
operations (when index records are physically deleted). An update
operation is represented as a combination of an insert and a
delete. The default value is
The allowed values of
The default value: buffer inserts, delete-marking operations, and purges.
Do not buffer any operations.
Buffer insert operations.
Buffer delete-marking operations.
Buffer both inserts and delete-marking.
Buffer the physical deletion operations that happen in the background.
You can set the value of this parameter in the MySQL option file
my.ini) or change
it dynamically with the
SET GLOBAL command,
which requires the
SUPER privilege. Changing
the setting affects the buffering of new operations; the merging
of already buffered entries is not affected.
For more information about speeding up
statements, see Section 8.2.2, “Optimizing DML Statements”.
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