MySQL 5.7 Release Notes
Reading rows using a range scan on a secondary index can result in many random disk accesses to the base table when the table is large and not stored in the storage engine's cache. With the Disk-Sweep Multi-Range Read (MRR) optimization, MySQL tries to reduce the number of random disk access for range scans by first scanning the index only and collecting the keys for the relevant rows. Then the keys are sorted and finally the rows are retrieved from the base table using the order of the primary key. The motivation for Disk-sweep MRR is to reduce the number of random disk accesses and instead achieve a more sequential scan of the base table data.
The Multi-Range Read optimization provides these benefits:
MRR enables data rows to be accessed sequentially rather than in random order, based on index tuples. The server obtains a set of index tuples that satisfy the query conditions, sorts them according to data row ID order, and uses the sorted tuples to retrieve data rows in order. This makes data access more efficient and less expensive.
MRR enables batch processing of requests for key access for operations that require access to data rows through index tuples, such as range index scans and equi-joins that use an index for the join attribute. MRR iterates over a sequence of index ranges to obtain qualifying index tuples. As these results accumulate, they are used to access the corresponding data rows. It is not necessary to acquire all index tuples before starting to read data rows.
The MRR optimization is not supported with secondary indexes
created on virtual generated columns.
InnoDB supports secondary indexes on
virtual generated columns.
The following scenarios illustrate when MRR optimization can be advantageous:
Scenario A: MRR can be used for InnoDB and
MyISAM tables for index range scans and
equi-join operations.
A portion of the index tuples are accumulated in a buffer.
The tuples in the buffer are sorted by their data row ID.
Data rows are accessed according to the sorted index tuple sequence.
Scenario B: MRR can be used for
NDB tables for multiple-range
index scans or when performing an equi-join by an attribute.
A portion of ranges, possibly single-key ranges, is accumulated in a buffer on the central node where the query is submitted.
The ranges are sent to the execution nodes that access data rows.
The accessed rows are packed into packages and sent back to the central node.
The received packages with data rows are placed in a buffer.
Data rows are read from the buffer.
When MRR is used, the Extra column in
EXPLAIN output shows
Using MRR.
InnoDB and MyISAM do not
use MRR if full table rows need not be accessed to produce the
query result. This is the case if results can be produced
entirely on the basis on information in the index tuples
(through a covering
index); MRR provides no benefit.
Two optimizer_switch system
variable flags provide an interface to the use of MRR
optimization. The mrr flag
controls whether MRR is enabled. If
mrr is enabled
(on), the
mrr_cost_based flag controls
whether the optimizer attempts to make a cost-based choice
between using and not using MRR (on) or
uses MRR whenever possible (off). By
default, mrr is
on and
mrr_cost_based is
on. See
Section 8.9.2, “Switchable Optimizations”.
For MRR, a storage engine uses the value of the
read_rnd_buffer_size system
variable as a guideline for how much memory it can allocate
for its buffer. The engine uses up to
read_rnd_buffer_size bytes
and determines the number of ranges to process in a single
pass.