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MySQL 5.6 Reference Manual  /  ...  /  Optimizing Subqueries with Semijoin Transformations Optimizing Subqueries with Semijoin Transformations

A semijoin is a preparation-time transformation that enables multiple execution strategies such as table pullout, duplicate weedout, first match, loose scan, and materialization. The optimizer uses semijoin strategies to improve subquery execution, as described in this section.

For an inner join between two tables, the join returns a row from one table as many times as there are matches in the other table. But for some questions, the only information that matters is whether there is a match, not the number of matches. Suppose that there are tables named class and roster that list classes in a course curriculum and class rosters (students enrolled in each class), respectively. To list the classes that actually have students enrolled, you could use this join:

SELECT class.class_num, class.class_name
FROM class INNER JOIN roster
WHERE class.class_num = roster.class_num;

However, the result lists each class once for each enrolled student. For the question being asked, this is unnecessary duplication of information.

Assuming that class_num is a primary key in the class table, duplicate suppression is possible by using SELECT DISTINCT, but it is inefficient to generate all matching rows first only to eliminate duplicates later.

The same duplicate-free result can be obtained by using a subquery:

SELECT class_num, class_name
FROM class
WHERE class_num IN (SELECT class_num FROM roster);

Here, the optimizer can recognize that the IN clause requires the subquery to return only one instance of each class number from the roster table. In this case, the query can use a semijoin; that is, an operation that returns only one instance of each row in class that is matched by rows in roster.

Outer join and inner join syntax is permitted in the outer query specification, and table references may be base tables, derived tables, or view references.

In MySQL, a subquery must satisfy these criteria to be handled as a semijoin:

  • It must be an IN (or =ANY) subquery that appears at the top level of the WHERE or ON clause, possibly as a term in an AND expression. For example:

    SELECT ...
    FROM ot1, ...
    WHERE (oe1, ...) IN (SELECT ie1, ... FROM it1, ... WHERE ...);

    Here, ot_i and it_i represent tables in the outer and inner parts of the query, and oe_i and ie_i represent expressions that refer to columns in the outer and inner tables.

  • It must be a single SELECT without UNION constructs.

  • It must not contain a GROUP BY or HAVING clause.

  • It must not be implicitly grouped (it must contain no aggregate functions).

  • It must not have ORDER BY with LIMIT.

  • The STRAIGHT_JOIN modifier must not be present.

  • The number of outer and inner tables together must be less than the maximum number of tables permitted in a join.

The subquery may be correlated or uncorrelated. DISTINCT is permitted, as is LIMIT unless ORDER BY is also used.

If a subquery meets the preceding criteria, MySQL converts it to a semijoin and makes a cost-based choice from these strategies:

  • Convert the subquery to a join, or use table pullout and run the query as an inner join between subquery tables and outer tables. Table pullout pulls a table out from the subquery to the outer query.

  • Duplicate Weedout: Run the semijoin as if it was a join and remove duplicate records using a temporary table.

  • FirstMatch: When scanning the inner tables for row combinations and there are multiple instances of a given value group, choose one rather than returning them all. This "shortcuts" scanning and eliminates production of unnecessary rows.

  • LooseScan: Scan a subquery table using an index that enables a single value to be chosen from each subquery's value group.

  • Materialize the subquery into an indexed temporary table that is used to perform a join, where the index is used to remove duplicates. The index might also be used later for lookups when joining the temporary table with the outer tables; if not, the table is scanned. For more information about materialization, see Section, “Optimizing Subqueries with Materialization”.

Each of these strategies except Duplicate Weedout can be enabled or disabled using the optimizer_switch system variable:

These flags are enabled by default. See Section 8.9.2, “Switchable Optimizations”.

EXPLAIN output indicates the use of semijoin strategies as follows:

  • Semijoined tables show up in the outer select. For extended EXPLAIN output, the text displayed by a following SHOW WARNINGS shows the rewritten query, which displays the semijoin structure. (See Section 8.8.3, “Extended EXPLAIN Output Format”.) From this you can get an idea about which tables were pulled out of the semijoin. If a subquery was converted to a semijoin, you can see that the subquery predicate is gone and its tables and WHERE clause were merged into the outer query join list and WHERE clause.

  • Temporary table use for Duplicate Weedout is indicated by Start temporary and End temporary in the Extra column. Tables that were not pulled out and are in the range of EXPLAIN output rows covered by Start temporary and End temporary have their rowid in the temporary table.

  • FirstMatch(tbl_name) in the Extra column indicates join shortcutting.

  • LooseScan(m..n) in the Extra column indicates use of the LooseScan strategy. m and n are key part numbers.

  • Temporary table use for materialization is indicated by rows with a select_type value of MATERIALIZED and rows with a table value of <subqueryN>.