(mysql.info) optimizing-subqueries

 
 13.2.8.10 Optimizing Subqueries
 ...............................
 
 Development is ongoing, so no optimization tip is reliable for the long
 term. The following list provides some interesting tricks that you
 might want to play with:
 
    * Use subquery clauses that affect the number or order of the rows
      in the subquery. For example:
 
           SELECT * FROM t1 WHERE t1.column1 IN
           (SELECT column1 FROM t2 ORDER BY column1);
           SELECT * FROM t1 WHERE t1.column1 IN
           (SELECT DISTINCT column1 FROM t2);
           SELECT * FROM t1 WHERE EXISTS
           (SELECT * FROM t2 LIMIT 1);
 
    * Replace a join with a subquery. For example, try this:
 
           SELECT DISTINCT column1 FROM t1 WHERE t1.column1 IN (
           SELECT column1 FROM t2);
 
      Instead of this:
 
           SELECT DISTINCT t1.column1 FROM t1, t2
           WHERE t1.column1 = t2.column1;
 
    * Some subqueries can be transformed to joins for compatibility with
      older versions of MySQL that do not support subqueries. However,
      in some cases, converting a subquery to a join may improve
      performance. See  rewriting-subqueries.
 
    * Move clauses from outside to inside the subquery. For example, use
      this query:
 
           SELECT * FROM t1
           WHERE s1 IN (SELECT s1 FROM t1 UNION ALL SELECT s1 FROM t2);
 
      Instead of this query:
 
           SELECT * FROM t1
           WHERE s1 IN (SELECT s1 FROM t1) OR s1 IN (SELECT s1 FROM t2);
 
      For another example, use this query:
 
           SELECT (SELECT column1 + 5 FROM t1) FROM t2;
 
      Instead of this query:
 
           SELECT (SELECT column1 FROM t1) + 5 FROM t2;
 
    * Use a row subquery instead of a correlated subquery. For example,
      use this query:
 
           SELECT * FROM t1
           WHERE (column1,column2) IN (SELECT column1,column2 FROM t2);
 
      Instead of this query:
 
           SELECT * FROM t1
           WHERE EXISTS (SELECT * FROM t2 WHERE t2.column1=t1.column1
           AND t2.column2=t1.column2);
 
    * Use `NOT (a = ANY (...))' rather than `a <> ALL (...)'.
 
    * Use `x = ANY (TABLE CONTAINING (1,2))' rather than `x=1 OR x=2'.
 
    * Use `= ANY' rather than `EXISTS'.
 
    * For uncorrelated subqueries that always return one row, `IN' is
      always slower than `='. For example, use this query:
 
           SELECT * FROM t1 WHERE t1.COL_NAME
           = (SELECT a FROM t2 WHERE b = SOME_CONST);
 
      Instead of this query:
 
           SELECT * FROM t1 WHERE t1.COL_NAME
           IN (SELECT a FROM t2 WHERE b = SOME_CONST);
 
 These tricks might cause programs to go faster or slower.  Using MySQL
 facilities like the `BENCHMARK()' function, you can get an idea about
 what helps in your own situation. See  information-functions.
 
 Some optimizations that MySQL itself makes are:
 
    * MySQL executes non-correlated subqueries only once. Use `EXPLAIN'
      to make sure that a given subquery really is non-correlated.
 
    * MySQL rewrites `IN', `ALL', `ANY', and `SOME' subqueries in an
      attempt to take advantage of the possibility that the select-list
      columns in the subquery are indexed.
 
    * MySQL replaces subqueries of the following form with an
      index-lookup function, which `EXPLAIN' describes as a special join
      type (`unique_subquery' or `index_subquery'):
 
           ... IN (SELECT INDEXED_COLUMN FROM SINGLE_TABLE ...)
 
    * MySQL enhances expressions of the following form with an
      expression involving `MIN()' or `MAX()', unless `NULL' values or
      empty sets are involved:
 
           VALUE {ALL|ANY|SOME} {> | < | >= | <=} (NON-CORRELATED SUBQUERY)
 
      For example, this `WHERE' clause:
 
           WHERE 5 > ALL (SELECT x FROM t)
 
      might be treated by the optimizer like this:
 
           WHERE 5 > (SELECT MAX(x) FROM t)
 
 There is a chapter titled `How MySQL Transforms Subqueries' in the
 MySQL Internals Manual, available at `http://dev.mysql.com/doc/'.
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