Optimizing LIMIT Queries

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 row_count clause and no HAVING clause:

• If you select only a few rows with LIMIT, MariaDB uses indexes in some cases when normally it would prefer to do a full table scan.
• If you use LIMIT row_count with ORDER BY, MariaDB ends the sorting as soon as it has found the first row_count 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 row_count are found. After the initial rows have been found, MariaDB does not sort any remainder of the result set.
• When combining LIMIT row_count with DISTINCT, MariaDB stops as soon as it finds row_count unique rows.
• In some cases, a GROUP BY can be resolved by reading the key in order (or doing a sort on the key) and then calculating summaries until the key value changes. In this case, LIMIT row_count does not calculate any unnecessary GROUP BY values.
• As soon as MariaDB has sent the required number of rows to the client, it aborts the query unless you are using SQL_CALC_FOUND_ROWS.
• LIMIT 0 quickly returns an empty set. This can be useful for checking the validity of a query. When using one of the MariaDB APIs, it can also be employed for obtaining the types of the result columns. (This trick does not work in the MariaDB Monitor (the mysql program), which merely displays Empty set in such cases; instead, use SHOW COLUMNS or DESCRIBE for this purpose.)
• When the server uses temporary tables to resolve the query, it uses the LIMIT row_count clause to calculate how much space is required.

As of MariaDB 5.6.2, the optimizer more efficiently handles queries (and subqueries) of the following form:

SELECT ... FROM single_table ... ORDER BY non_index_column [DESC] LIMIT [M,]N;

That type of query is common in web applications that display only a few rows from a larger result set. For example:

SELECT col1, ... FROM t1 ... ORDER BY name LIMIT 10;
SELECT col1, ... FROM t1 ... ORDER BY RAND() LIMIT 15;

The sort buffer has a size of sort_buffer_size. If the sort elements for N rows are small enough to fit in the sort buffer (M+N rows if M was specified), the server can avoid using a merge file and perform the sort entirely in memory by treating the sort buffer as a priority queue:

• Scan the table, inserting the select list columns from each selected row in sorted order in the queue. If the queue is full, bump out the last row in the sort order.
• Return the first N rows from the queue. (If M was specified, skip the first M rows and return the next N rows.)

Previously, the server performed this operation by using a merge file for the sort:

• Scan the table, repeating these steps through the end of the table:
  • Select rows until the sort buffer is filled.
  • Write the first N rows in the buffer (M+N rows if M was specified) to a merge file.
• Sort the merge file and return the first N rows. (If M was specified, skip the first M rows and return the next N rows.)

The cost of the table scan is the same for the queue and merge-file methods, so the optimizer chooses between methods based on other costs:

• The queue method involves more CPU for inserting rows into the queue in order
• The merge-file method has I/O costs to write and read the file and CPU cost to sort it

The optimizer considers the balance between these factors for particular values of N and the row size.

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