#### 8.2.1.1 WHERE Clause Optimization

[]()[]()

 This section discusses optimizations that can be made for processing `WHERE` clauses. The examples use [`SELECT`](select.html) statements, but the same optimizations apply for `WHERE` clauses in [`DELETE`](delete.html) and [`UPDATE`](update.html) statements.

Note

 Because work on the MySQL optimizer is ongoing, not all of the optimizations that MySQL performs are documented here.

 You might be tempted to rewrite your queries to make arithmetic operations faster, while sacrificing readability. Because MySQL does similar optimizations automatically, you can often avoid this work, and leave the query in a more understandable and maintainable form. 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:

  ```
     (b>=5 AND b=5) OR (b=6 AND 5=5) OR (b=7 AND 5=6)
  -> b=5 OR b=6
  ```

   In MySQL 8.0.14 and later, this takes place during preparation rather than during the optimization phase, which helps in simplification of joins. See [Section 8.2.1.9, “Outer Join Optimization”](outer-join-optimization.html), for further information and examples.

* Constant expressions used by indexes are evaluated only once.

* Beginning with MySQL 8.0.16, comparisons of columns of numeric types with constant values are checked and folded or removed for invalid or out-of-rage values:

  ```
  # CREATE TABLE t (c TINYINT UNSIGNED NOT NULL);
    SELECT * FROM t WHERE c ≪ 256;
  -≫ SELECT * FROM t WHERE 1;
  ```

   See [Section 8.2.1.14, “Constant-Folding Optimization”](constant-folding-optimization.html), for more information.

* [`COUNT(*)`](aggregate-functions.html#function_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`](select.html) statements are impossible and returns no rows.

* `HAVING` is merged with `WHERE` if you do not use `GROUP BY` or aggregate functions ([`COUNT()`](aggregate-functions.html#function_count), [`MIN()`](aggregate-functions.html#function_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` modifier, 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 MAX(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, ... ;
```

