MySQL 8.0 Reference Manual Including MySQL NDB Cluster 8.0

Comparisons between constants and column values in which the
constant value is out of range or of the wrong type with
respect to the column type are now handled once during query
optimization rather row-by-row than during execution. The
comparisons that can be treated in this manner are
`>`

, `>=`

,
`<`

, `<=`

,
`<>`

/`!=`

,
`=`

, and `<=>`

.

Consider the table created by the following statement:

CREATE TABLE t (c TINYINT UNSIGNED NOT NULL);

The `WHERE`

condition in the query
`SELECT * FROM t WHERE c < 256`

contains
the integral constant 256 which is out of range for a
`TINYINT UNSIGNED`

column. Previously, this
was handled by treating both operands as the larger type, but
now, since any allowed value for `c`

is less
than the constant, the `WHERE`

expression can
instead be folded as `WHERE 1`

, so that the
query is rewritten as ```
SELECT * FROM t WHERE
1
```

.

This makes it possible for the optimizer to remove the
`WHERE`

expression altogether. If the column
`c`

were nullable (that is, defined only as
`TINYINT UNSIGNED`

) the query would be
rewritten like this:

SELECT * FROM t WHERE ti IS NOT NULL

Folding is performed for constants compared to supported MySQL column types as follows:

**Integer column type.**Integer types are compared with constants of the following types as described here:**Integer value.**If the constant is out of range for the column type, the comparison is folded to`1`

or`IS NOT NULL`

, as already shown.If the constant is a range boundary, the comparison is folded to

`=`

. For example (using the same table as already defined):mysql>

*************************** 1. row *************************** id: 1 select_type: SIMPLE table: t partitions: NULL type: ALL possible_keys: NULL key: NULL key_len: NULL ref: NULL rows: 5 filtered: 20.00 Extra: Using where 1 row in set, 1 warning (0.00 sec) mysql>`EXPLAIN SELECT * FROM t WHERE c >= 255;`

*************************** 1. row *************************** Level: Note Code: 1003 Message: /* select#1 */ select `test`.`t`.`ti` AS `ti` from `test`.`t` where (`test`.`t`.`ti` = 255) 1 row in set (0.00 sec)`SHOW WARNINGS;`

**Floating- or fixed-point value.**If the constant is one of the decimal types (such as`DECIMAL`

,`REAL`

,`DOUBLE`

, or`FLOAT`

) and has a nonzero decimal portion, it cannot be equal; fold accordingly. For other comparisons, round up or down to an integer value according to the sign, then perform a range check and handle as already described for integer-integer comparisons.A

`REAL`

value that is too small to be represented as`DECIMAL`

is rounded to .01 or -.01 depending on the sign, then handled as a`DECIMAL`

.**String types.**Try to interpret the string value as an integer type, then handle the comparison as between integer values. If this fails, attempt to handle the value as a`REAL`

.

**DECIMAL or REAL column.**Decimal types are compared with constants of the following types as described here:**Integer value.**Perform a range check against the column value's integer part. If no folding results, convert the constant to`DECIMAL`

with the same number of decimal places as the column value, then check it as a`DECIMAL`

(see next).**DECIMAL or REAL value.**Check for overflow (that is, whether the constant has more digits in its integer part than allowed for the column's decimal type). If so, fold.If the constant has more significant fractional digits than column's type, truncate the constant. If the comparison operator is

`=`

or`<>`

, fold. If the operator is`>=`

or`<=`

, adjust the operator due to truncation. For example, if column's type is`DECIMAL(3,1)`

,`SELECT * FROM t WHERE f >= 10.13`

becomes`SELECT * FROM t WHERE f > 10.1`

.If the constant has fewer decimal digits than the column's type, convert it to a constant with same number of digits. For underflow of a

`REAL`

value (that is, too few fractional digits to represent it), convert the constant to decimal 0.**String value.**If the value can be interpreted as an integer type, handle it as such. Otherwise, try to handle it as`REAL`

.

**FLOAT or DOUBLE column.**`FLOAT(`

or,`m`

)`n`

`DOUBLE(`

values compared with constants are handled as follows:,`m`

)`n`

If the value overflows the range of the column, fold.

If the value has more than

decimals, truncate, compensating during folding. For`n`

`=`

and`<>`

comparisons, fold to`TRUE`

,`FALSE`

, or`IS [NOT] NULL`

as described previously; for other operators, adjust the operator.If the value has more than

`m`

integer digits, fold.

**Limitations. **
This optimization cannot be used in the following cases:

With comparisons using

`BETWEEN`

or`IN`

.With

`BIT`

columns or columns using date or time types.During the preparation phase for a prepared statement, although it can be applied during the optimization phase when the prepared statement is actually executed. This due to the fact that, during statement preparation, the value of the constant is not yet known.