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(mysql.info)Where optimizations

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How MySQL Optimizes `WHERE' Clauses
-----------------------------------

The `WHERE' optimizations are put in the `SELECT' part here because
they are mostly used with `SELECT', but the same optimizations apply for
`WHERE' in `DELETE' and `UPDATE' statements.

Also note that this section is incomplete. MySQL does many
optimizations, and we have not had time to document them all.

Some of the optimizations performed by MySQL are listed below:

   * Removal of unnecessary parentheses:
             ((a AND b) AND c OR (((a AND b) AND (c AND d))))
          -> (a AND b AND c) OR (a AND b AND c AND d)

   * Constant folding:
             (a<b AND b=c) AND a=5
          -> b>5 AND b=c AND a=5

   * Constant condition removal (needed because of constant folding):
             (B>=5 AND B=5) OR (B=6 AND 5=5) OR (B=7 AND 5=6)
          -> B=5 OR B=6

   * Constant expressions used by indexes are evaluated only once.

   * `COUNT(*)' on a single table without a `WHERE' is retrieved
     directly from the table information.  This is also done for any
     `NOT NULL' expression when used with only one table.

   * Early detection of invalid constant expressions. MySQL quickly
     detects that some `SELECT' statements are impossible and returns
     no rows.

   * `HAVING' is merged with `WHERE' if you don't use `GROUP BY' or
     group functions (`COUNT()', `MIN()'...).

   * For each sub-join, a simpler `WHERE' is constructed to get a fast
     `WHERE' evaluation for each sub-join and also to skip records as
     soon as possible.

   * All constant tables are read first, before any other tables in the
     query.  A constant table is:
        - An empty table or a table with 1 row.

        - A table that is used with a `WHERE' clause on a `UNIQUE'
          index, or a `PRIMARY KEY', where all index parts are used
          with constant expressions and the index parts are defined as
          `NOT NULL'.
     All the following tables are used as constant tables:
          mysql> SELECT * FROM t WHERE primary_key=1;
          mysql> SELECT * FROM t1,t2
                     WHERE t1.primary_key=1 AND t2.primary_key=t1.id;

   * The best join combination to join the tables is found by trying all
     possibilities. If all columns in `ORDER BY' and in `GROUP BY' come
     from the same table, then this table is preferred first when
     joining.

   * If there is an `ORDER BY' clause and a different `GROUP BY'
     clause, or if the `ORDER BY' or `GROUP BY' contains columns from
     tables other than the first table in the join queue, a temporary
     table is created.

   * If you use `SQL_SMALL_RESULT', MySQL will use an in-memory
     temporary table.

   * Each table index is queried, and the best index that spans fewer
     than 30% of the rows is used. If no such index can be found, a
     quick table scan is used.

   * In some cases, MySQL can read rows from the index without even
     consulting the data file.  If all columns used from the index are
     numeric, then only the index tree is used to resolve the query.

   * Before each record is output, those that do not match the `HAVING'
     clause are skipped.

Some examples of queries that are very fast:

     mysql> SELECT COUNT(*) FROM tbl_name;
     mysql> SELECT MIN(key_part1),MAX(key_part1) FROM tbl_name;
     mysql> SELECT MAX(key_part2) FROM tbl_name
                WHERE key_part_1=constant;
     mysql> SELECT ... FROM tbl_name
                ORDER BY key_part1,key_part2,... LIMIT 10;
     mysql> SELECT ... FROM tbl_name
                ORDER BY key_part1 DESC,key_part2 DESC,... LIMIT 10;

The following queries are resolved using only the index tree (assuming
the indexed columns are numeric):

     mysql> SELECT key_part1,key_part2 FROM tbl_name WHERE key_part1=val;
     mysql> SELECT COUNT(*) FROM tbl_name
                WHERE key_part1=val1 AND key_part2=val2;
     mysql> SELECT key_part2 FROM tbl_name GROUP BY key_part1;

The following queries use indexing to retrieve the rows in sorted order
without a separate sorting pass:

     mysql> SELECT ... FROM tbl_name ORDER BY key_part1,key_part2,... ;
     mysql> SELECT ... FROM tbl_name ORDER BY key_part1 DESC,key_part2 DESC,... ;