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 7.2.4 `WHERE' Clause Optimization
 ---------------------------------
 
 This section discusses optimizations that can be made for processing
 `WHERE' clauses. The examples use `SELECT' statements, but the same
 optimizations apply for `WHERE' clauses in `DELETE' and `UPDATE'
 statements.
 
 Work on the MySQL optimizer is ongoing, so this section is incomplete.
 MySQL performs a great many optimizations, not all of which are
 documented here.
 
 Some of the optimizations performed by MySQL follow:
 
    * 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 for `MyISAM' and `MEMORY'
      tables. 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 do not use `GROUP BY' or
      aggregate functions (`COUNT()', `MIN()', and so on).
 
    * For each table in a join, a simpler `WHERE' is constructed to get
      a fast `WHERE' evaluation for the table and also to skip rows as
      soon as possible.
 
    * All constant tables are read first before any other tables in the
      query. A constant table is any of the following:
 
         * An empty table or a table with one row.
 
         * A table that is used with a `WHERE' clause on a `PRIMARY KEY'
           or a `UNIQUE' index, where all index parts are compared to
           constant expressions and are defined as `NOT NULL'.
 
      All of the following tables are used as constant tables:
 
           SELECT * FROM t WHERE PRIMARY_KEY=1;
           SELECT * FROM t1,t2
             WHERE t1.PRIMARY_KEY=1 AND t2.PRIMARY_KEY=t1.id;
 
    * The best join combination for joining the tables is found by
      trying all possibilities. If all columns in `ORDER BY' and `GROUP
      BY' clauses come from the same table, that 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 the `SQL_SMALL_RESULT' option, MySQL uses an in-memory
      temporary table.
 
    * Each table index is queried, and the best index is used unless the
      optimizer believes that it is more efficient to use a table scan.
      At one time, a scan was used based on whether the best index
      spanned more than 30% of the table, but a fixed percentage no
      longer determines the choice between using an index or a scan. The
      optimizer now is more complex and bases its estimate on additional
      factors such as table size, number of rows, and I/O block size.
 
    * 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, only the index tree is used to resolve the query.
 
    * Before each row is output, those that do not match the `HAVING'
      clause are skipped.
 
 Some examples of queries that are very fast:
 
      SELECT COUNT(*) FROM TBL_NAME;
 
      SELECT MIN(KEY_PART1),MAX(KEY_PART1) FROM TBL_NAME;
 
      SELECT MAX(KEY_PART2) FROM TBL_NAME
        WHERE KEY_PART1=CONSTANT;
 
      SELECT ... FROM TBL_NAME
        ORDER BY KEY_PART1,KEY_PART2,... LIMIT 10;
 
      SELECT ... FROM TBL_NAME
        ORDER BY KEY_PART1 DESC, KEY_PART2 DESC, ... LIMIT 10;
 
 MySQL resolves the following queries using only the index tree,
 assuming that the indexed columns are numeric:
 
      SELECT KEY_PART1,KEY_PART2 FROM TBL_NAME WHERE KEY_PART1=VAL;
 
      SELECT COUNT(*) FROM TBL_NAME
        WHERE KEY_PART1=VAL1 AND KEY_PART2=VAL2;
 
      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:
 
      SELECT ... FROM TBL_NAME
        ORDER BY KEY_PART1,KEY_PART2,... ;
 
      SELECT ... FROM TBL_NAME
        ORDER BY KEY_PART1 DESC, KEY_PART2 DESC, ... ;
 

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