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This section describes when MySQL can use an index to satisfy
an ORDER BY clause, the
filesort operation used when an index
cannot be used, and execution plan information available from
the optimizer about ORDER BY.
An ORDER BY with and without
LIMIT may return rows in different orders,
as discussed in Section 8.2.1.19, “LIMIT Query Optimization”.
Use of Indexes to Satisfy ORDER BY
Use of filesort to Satisfy ORDER BY
Influencing ORDER BY Optimization
ORDER BY Execution Plan Information Available
In some cases, MySQL may use an index to satisfy an
ORDER BY clause and avoid the extra
sorting involved in performing a filesort
operation.
The index may also be used even if the ORDER
BY does not match the index exactly, as long as
all unused portions of the index and all extra
ORDER BY columns are constants in the
WHERE clause. If the index does not
contain all columns accessed by the query, the index is used
only if index access is cheaper than other access methods.
Assuming that there is an index on
(key_part1,
key_part2), the
following queries may use the index to resolve the
ORDER BY part. Whether the optimizer
actually does so depends on whether reading the index is
more efficient than a table scan if columns not in the index
must also be read.
In this query, the index on
(key_part1,
key_part2) enables
the optimizer to avoid sorting:
SELECT * FROM t1
ORDER BY key_part1, key_part2;
However, the query uses SELECT *,
which may select more columns than
key_part1 and
key_part2. In that case,
scanning an entire index and looking up table rows to
find columns not in the index may be more expensive than
scanning the table and sorting the results. If so, the
optimizer probably does not use the index. If
SELECT * selects only the index
columns, the index is used and sorting avoided.
If t1 is an InnoDB
table, the table primary key is implicitly part of the
index, and the index can be used to resolve the
ORDER BY for this query:
SELECT pk, key_part1, key_part2 FROM t1
ORDER BY key_part1, key_part2;
In this query, key_part1 is
constant, so all rows accessed through the index are in
key_part2 order, and an index
on (key_part1,
key_part2) avoids
sorting if the WHERE clause is
selective enough to make an index range scan cheaper
than a table scan:
SELECT * FROM t1
WHERE key_part1 = constant
ORDER BY key_part2;
In the next two queries, whether the index is used is
similar to the same queries without
DESC shown previously:
SELECT * FROM t1
ORDER BY key_part1 DESC, key_part2 DESC;
SELECT * FROM t1
WHERE key_part1 = constant
ORDER BY key_part2 DESC;
Two columns in an ORDER BY can sort
in the same direction (both ASC, or
both DESC) or in opposite directions
(one ASC, one
DESC). A condition for index use is
that the index must have the same homogeneity, but need
not have the same actual direction.
If a query mixes ASC and
DESC, the optimizer can use an index
on the columns if the index also uses corresponding
mixed ascending and descending columns:
SELECT * FROM t1
ORDER BY key_part1 DESC, key_part2 ASC;
The optimizer can use an index on
(key_part1,
key_part2) if
key_part1 is descending and
key_part2 is ascending. It
can also use an index on those columns (with a backward
scan) if key_part1 is
ascending and key_part2 is
descending. See Section 8.3.13, “Descending Indexes”.
In the next two queries,
key_part1 is compared to a
constant. The index is used if the
WHERE clause is selective enough to
make an index range scan cheaper than a table scan:
SELECT * FROM t1
WHERE key_part1 constant
ORDER BY key_part1 ASC;
SELECT * FROM t1
WHERE key_part1 constant
ORDER BY key_part1 DESC;
In the next query, the ORDER BY does
not name key_part1, but all
rows selected have a constant
key_part1 value, so the index
can still be used:
SELECT * FROM t1
WHERE key_part1 = constant1 AND key_part2 constant2
ORDER BY key_part2;
In some cases, MySQL cannot use indexes
to resolve the ORDER BY, although it may
still use indexes to find the rows that match the
WHERE clause. Examples:
The query uses ORDER BY on different
indexes:
SELECT * FROM t1 ORDER BY key1, key2;
The query uses ORDER BY on
nonconsecutive parts of an index:
SELECT * FROM t1 WHERE key2=constant ORDER BY key1_part1, key1_part3;
The index used to fetch the rows differs from the one
used in the ORDER BY:
SELECT * FROM t1 WHERE key2=constant ORDER BY key1;
The query uses ORDER BY with an
expression that includes terms other than the index
column name:
SELECT * FROM t1 ORDER BY ABS(key);
SELECT * FROM t1 ORDER BY -key;
The query joins many tables, and the columns in the
ORDER BY are not all from the first
nonconstant table that is used to retrieve rows. (This
is the first table in the
EXPLAIN output that does
not have a const join
type.)
The query has different ORDER BY and
GROUP BY expressions.
There is an index on only a prefix of a column named in
the ORDER BY clause. In this case,
the index cannot be used to fully resolve the sort
order. For example, if only the first 10 bytes of a
CHAR(20) column are
indexed, the index cannot distinguish values past the
10th byte and a filesort is needed.
The index does not store rows in order. For example,
this is true for a HASH index in a
MEMORY table.
Availability of an index for sorting may be affected by the
use of column aliases. Suppose that the column
t1.a is indexed. In this statement, the
name of the column in the select list is
a. It refers to t1.a,
as does the reference to a in the
ORDER BY, so the index on
t1.a can be used:
SELECT a FROM t1 ORDER BY a;
In this statement, the name of the column in the select list
is also a, but it is the alias name. It
refers to ABS(a), as does the reference
to a in the ORDER BY,
so the index on t1.a cannot be used:
SELECT ABS(a) AS a FROM t1 ORDER BY a;
In the following statement, the ORDER BY
refers to a name that is not the name of a column in the
select list. But there is a column in t1
named a, so the ORDER
BY refers to t1.a and the index
on t1.a can be used. (The resulting sort
order may be completely different from the order for
ABS(a), of course.)
SELECT ABS(a) AS b FROM t1 ORDER BY a;
Previously (MySQL 5.7 and lower),
GROUP BY sorted implicitly under certain
conditions. In MySQL 8.0, that no longer
occurs, so specifying ORDER BY NULL at
the end to suppress implicit sorting (as was done
previously) is no longer necessary. However, query results
may differ from previous MySQL versions. To produce a given
sort order, provide an ORDER BY clause.
If an index cannot be used to satisfy an ORDER
BY clause, MySQL performs a
filesort operation that reads table rows
and sorts them. A filesort constitutes an
extra sorting phase in query execution.
To obtain memory for filesort operations,
as of MySQL 8.0.12, the optimizer allocates memory buffers
incrementally as needed, up to the size indicated by the
sort_buffer_size system
variable, rather than allocating a fixed amount of
sort_buffer_size bytes up
front, as was done prior to MySQL 8.0.12. This enables users
to set sort_buffer_size to
larger values to speed up larger sorts, without concern for
excessive memory use for small sorts. (This benefit may not
occur for multiple concurrent sorts on Windows, which has a
weak multithreaded malloc.)
A filesort operation uses temporary disk
files as necessary if the result set is too large to fit in
memory. Some types of queries are particularly suited to
completely in-memory filesort operations.
For example, the optimizer can use
filesort to efficiently handle in memory,
without temporary files, the ORDER BY
operation for queries (and subqueries) of the following
form:
SELECT ... FROM single_table ... ORDER BY non_index_column [DESC] LIMIT [M,]N;
Such queries are common in web applications that display
only a few rows from a larger result set. Examples:
SELECT col1, ... FROM t1 ... ORDER BY name LIMIT 10;
SELECT col1, ... FROM t1 ... ORDER BY RAND() LIMIT 15;
filesort is not used, try lowering the
max_length_for_sort_data
system variable to a value that is appropriate to trigger a
filesort. (A symptom of setting the value
of this variable too high is a combination of high disk
activity and low CPU activity.) This technique applies only
before MySQL 8.0.20. As of 8.0.20,
max_length_for_sort_data is
deprecated due to optimizer changes that make it obsolete
and of no effect.
To increase ORDER BY speed, check whether
you can get MySQL to use indexes rather than an extra
sorting phase. If this is not possible, try the following
strategies:
Increase the
sort_buffer_size
variable value. Ideally, the value should be large
enough for the entire result set to fit in the sort
buffer (to avoid writes to disk and merge passes).
Take into account that the size of column values stored
in the sort buffer is affected by the
max_sort_length system
variable value. For example, if tuples store values of
long string columns and you increase the value of
max_sort_length, the
size of sort buffer tuples increases as well and may
require you to increase
sort_buffer_size.
To monitor the number of merge passes (to merge
temporary files), check the
Sort_merge_passes
status variable.
Increase the
read_rnd_buffer_size
variable value so that more rows are read at a time.
Change the tmpdir
system variable to point to a dedicated file system with
large amounts of free space. The variable value can list
several paths that are used in round-robin fashion; you
can use this feature to spread the load across several
directories. Separate the paths by colon characters
(:) on Unix and semicolon characters
(;) on Windows. The paths should name
directories in file systems located on different
physical disks, not different
partitions on the same disk.
ORDER BY Execution Plan Information Available
EXPLAIN
(see Section 8.8.1, “Optimizing Queries with EXPLAIN”), you can check whether
MySQL can use indexes to resolve an ORDER
BY clause:
If the Extra column of
EXPLAIN output does not
contain Using filesort, the index is
used and a filesort is not performed.
If the Extra column of
EXPLAIN output contains
Using filesort, the index is not used
and a filesort is performed.
In addition, if a filesort is performed,
optimizer trace output includes a
filesort_summary block. For example:
"filesort_summary": {
"rows": 100,
"examined_rows": 100,
"number_of_tmp_files": 0,
"peak_memory_used": 25192,
"sort_mode": " sort_key, packed_additional_fields "
}
peak_memory_used indicates the maximum
memory used at any one time during the sort. This is a value
up to but not necessarily as large as the value of the
sort_buffer_size system
variable. Prior to MySQL 8.0.12, the output shows
sort_buffer_size instead, indicating the
value of sort_buffer_size.
(Prior to MySQL 8.0.12, the optimizer always allocates
sort_buffer_size bytes for
the sort buffer. As of 8.0.12, the optimizer allocates
sort-buffer memory incrementally, beginning with a small
amount and adding more as necessary, up to
sort_buffer_size bytes.)
The sort_mode value provides information
about the contents of tuples in the sort buffer:
sort_key, rowid : This
indicates that sort buffer tuples are pairs that contain
the sort key value and row ID of the original table row.
Tuples are sorted by sort key value and the row ID is
used to read the row from the table.
sort_key, additional_fields :
This indicates that sort buffer tuples contain the sort
key value and columns referenced by the query. Tuples
are sorted by sort key value and column values are read
directly from the tuple.
sort_key,
packed_additional_fields : Like the
previous variant, but the additional columns are packed
tightly together instead of using a fixed-length
encoding.
EXPLAIN does not distinguish
whether the optimizer does or does not perform a
filesort in memory. Use of an in-memory
filesort can be seen in optimizer trace
output. Look for
filesort_priority_queue_optimization. For
information about the optimizer trace, see
MySQL Internals: Tracing the Optimizer.
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