MySQL :: MySQL 8.0 Reference Manual :: 10.2.1.19 LIMIT Query Optimization (original) (raw)

10.2.1.19 LIMIT Query Optimization

If you need only a specified number of rows from a result set, use a LIMIT clause in the query, rather than fetching the whole result set and throwing away the extra data.

MySQL sometimes optimizes a query that has a LIMIT_`rowcount`_ clause and noHAVING clause:

If you select only a few rows withLIMIT, MySQL uses indexes in some cases when normally it would prefer to do a full table scan.
If you combine LIMIT_`rowcount`_ withORDER BY, MySQL stops sorting as soon as it has found the first_rowcount_ rows of the sorted result, rather than sorting the entire result. If ordering is done by using an index, this is very fast. If a filesort must be done, all rows that match the query without the LIMIT clause are selected, and most or all of them are sorted, before the first_rowcount_ are found. After the initial rows have been found, MySQL does not sort any remainder of the result set.
One manifestation of this behavior is that anORDER BY query with and withoutLIMIT may return rows in different order, as described later in this section.
If you combine LIMIT_`rowcount`_ withDISTINCT, MySQL stops as soon as it finds rowcount unique rows.
In some cases, a GROUP BY can be resolved by reading the index in order (or doing a sort on the index), then calculating summaries until the index value changes. In this case, LIMIT_`rowcount`_ does not calculate any unnecessary GROUP BY values.
As soon as MySQL has sent the required number of rows to the client, it aborts the query unless you are usingSQL_CALC_FOUND_ROWS. In that case, the number of rows can be retrieved with SELECT FOUND_ROWS(). SeeSection 14.15, “Information Functions”.
LIMIT 0 quickly returns an empty set. This can be useful for checking the validity of a query. It can also be employed to obtain the types of the result columns within applications that use a MySQL API that makes result set metadata available. With themysql client program, you can use the--column-type-info option to display result column types.
If the server uses temporary tables to resolve a query, it uses the LIMIT_`rowcount`_ clause to calculate how much space is required.
If an index is not used for ORDER BY but a LIMIT clause is also present, the optimizer may be able to avoid using a merge file and sort the rows in memory using an in-memoryfilesort operation.

If multiple rows have identical values in the ORDER BY columns, the server is free to return those rows in any order, and may do so differently depending on the overall execution plan. In other words, the sort order of those rows is nondeterministic with respect to the nonordered columns.

One factor that affects the execution plan isLIMIT, so an ORDER BY query with and without LIMIT may return rows in different orders. Consider this query, which is sorted by the category column but nondeterministic with respect to the id andrating columns:

mysql> SELECT * FROM ratings ORDER BY category;
+----+----------+--------+
| id | category | rating |
+----+----------+--------+
|  1 |        1 |    4.5 |
|  5 |        1 |    3.2 |
|  3 |        2 |    3.7 |
|  4 |        2 |    3.5 |
|  6 |        2 |    3.5 |
|  2 |        3 |    5.0 |
|  7 |        3 |    2.7 |
+----+----------+--------+

Including LIMIT may affect order of rows within each category value. For example, this is a valid query result:

mysql> SELECT * FROM ratings ORDER BY category LIMIT 5;
+----+----------+--------+
| id | category | rating |
+----+----------+--------+
|  1 |        1 |    4.5 |
|  5 |        1 |    3.2 |
|  4 |        2 |    3.5 |
|  3 |        2 |    3.7 |
|  6 |        2 |    3.5 |
+----+----------+--------+

In each case, the rows are sorted by the ORDER BY column, which is all that is required by the SQL standard.

If it is important to ensure the same row order with and without LIMIT, include additional columns in the ORDER BY clause to make the order deterministic. For example, if id values are unique, you can make rows for a givencategory value appear inid order by sorting like this:

mysql> SELECT * FROM ratings ORDER BY category, id;
+----+----------+--------+
| id | category | rating |
+----+----------+--------+
|  1 |        1 |    4.5 |
|  5 |        1 |    3.2 |
|  3 |        2 |    3.7 |
|  4 |        2 |    3.5 |
|  6 |        2 |    3.5 |
|  2 |        3 |    5.0 |
|  7 |        3 |    2.7 |
+----+----------+--------+

mysql> SELECT * FROM ratings ORDER BY category, id LIMIT 5;
+----+----------+--------+
| id | category | rating |
+----+----------+--------+
|  1 |        1 |    4.5 |
|  5 |        1 |    3.2 |
|  3 |        2 |    3.7 |
|  4 |        2 |    3.5 |
|  6 |        2 |    3.5 |
+----+----------+--------+

For a query with an ORDER BY orGROUP BY and a LIMIT clause, the optimizer tries to choose an ordered index by default when it appears doing so would speed up query execution. Prior to MySQL 8.0.21, there was no way to override this behavior, even in cases where using some other optimization might be faster. Beginning with MySQL 8.0.21, it is possible to turn off this optimization by setting theoptimizer_switch system variable'sprefer_ordering_index flag to off.

Example: First we create and populate a table t as shown here:

# Create and populate a table t:

mysql> CREATE TABLE t (
    ->     id1 BIGINT NOT NULL,
    ->     id2 BIGINT NOT NULL,
    ->     c1 VARCHAR(50) NOT NULL,
    ->     c2 VARCHAR(50) NOT NULL,
    ->  PRIMARY KEY (id1),
    ->  INDEX i (id2, c1)
    -> );

# [Insert some rows into table t - not shown]

Verify that theprefer_ordering_index flag is enabled:

mysql> SELECT @@optimizer_switch LIKE '%prefer_ordering_index=on%';
+------------------------------------------------------+
| @@optimizer_switch LIKE '%prefer_ordering_index=on%' |
+------------------------------------------------------+
|                                                    1 |
+------------------------------------------------------+

Since the following query has a LIMIT clause, we expect it to use an ordered index, if possible. In this case, as we can see from theEXPLAIN output, it uses the table's primary key.

mysql> EXPLAIN SELECT c2 FROM t
    ->     WHERE id2 > 3
    ->     ORDER BY id1 ASC LIMIT 2\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: t
   partitions: NULL
         type: index
possible_keys: i
          key: PRIMARY
      key_len: 8
          ref: NULL
         rows: 2
     filtered: 70.00
        Extra: Using where

Now we disable theprefer_ordering_index flag, and re-run the same query; this time it uses the indexi (which includes theid2 column used in theWHERE clause), and a filesort:

mysql> SET optimizer_switch = "prefer_ordering_index=off";

mysql> EXPLAIN SELECT c2 FROM t
    ->     WHERE id2 > 3
    ->     ORDER BY id1 ASC LIMIT 2\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: t
   partitions: NULL
         type: range
possible_keys: i
          key: i
      key_len: 8
          ref: NULL
         rows: 14
     filtered: 100.00
        Extra: Using index condition; Using filesort