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MySQL 9.1 Reference Manual

10.2.1.14 Constant-Folding Optimization

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);

TheWHERE condition in the querySELECT * FROM t WHERE c < 256 contains the integral constant 256 which is out of range for aTINYINT UNSIGNED column. Previously, this was handled by treating both operands as the larger type, but now, since any allowed value forc is less than the constant, theWHERE expression can instead be folded asWHERE 1, so that the query is rewritten asSELECT * FROM t WHERE 1.

This makes it possible for the optimizer to remove theWHERE expression altogether. If the columnc were nullable (that is, defined only asTINYINT 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 to1 orIS 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> EXPLAIN SELECT * FROM t WHERE c >= 255;*************************** 1. row ***************************           id: 1  select_type: SIMPLE        table: t   partitions: NULL         type: ALLpossible_keys: NULL          key: NULL      key_len: NULL          ref: NULL         rows: 5     filtered: 20.00        Extra: Using where1 row in set, 1 warning (0.00 sec)mysql> SHOW WARNINGS;*************************** 1. row ***************************  Level: Note   Code: 1003Message: /* select#1 */ select `test`.`t`.`ti` AS `ti` from `test`.`t` where (`test`.`t`.`ti` = 255)1 row in set (0.00 sec)
```
- Floating- or fixed-point value. If the constant is one of the decimal types (such asDECIMAL,REAL,DOUBLE, orFLOAT) 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.
  AREAL value that is too small to be represented asDECIMAL is rounded to .01 or -.01 depending on the sign, then handled as aDECIMAL.
- 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 aREAL.
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 toDECIMAL with the same number of decimal places as the column value, then check it as aDECIMAL (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 isDECIMAL(3,1),SELECT * FROM t WHERE f >= 10.13 becomesSELECT * 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 aREAL 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 asREAL.
FLOAT or DOUBLE column. FLOAT(m,n) orDOUBLE(m,n) values compared with constants are handled as follows:
If the value overflows the range of the column, fold.
If the value has more thann decimals, truncate, compensating during folding. For= and<> comparisons, fold toTRUE,FALSE, orIS [NOT] NULL as described previously; for other operators, adjust the operator.
If the value has more thanm integer digits, fold.

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

With comparisons usingBETWEEN orIN.
WithBIT 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.

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