MySQL 8.4.0
Source Code Documentation

Enumerations  
enum  Range_placement { RP_OUTSIDE_HIGH , RP_OUTSIDE_LOW , RP_ON_MIN , RP_ON_MAX , RP_INSIDE , RP_INSIDE_TRUNCATED , RP_INSIDE_YEAR_HOLE , RP_ROUNDED_DOWN , RP_ROUNDED_UP } 
fold_condition uses analyze_field_constant to analyze constants in comparisons expressions with a view to fold the expression. More...  
Functions  
static bool  round_fold_or_convert_dec (THD *thd, Item **const_val, Range_placement *place, Item_field *f, my_decimal *d, bool up, bool *discount_equal) 
Minion of analyze_int_field_constant. More...  
static bool  fold_or_convert_dec (THD *thd, Item **const_val, Range_placement *place, Item_field *f, my_decimal *d) 
Minion of analyze_int_field_constant. More...  
static bool  analyze_int_field_constant (THD *thd, Item_field *f, Item **const_val, Item_func::Functype ft, bool left_has_field, Range_placement *place, bool *discount_equal) 
Minion of analyze_field_constant for integer type fields. More...  
static bool  analyze_decimal_field_constant (THD *thd, const Item_field *f, Item **const_val, Item_func::Functype ft, Range_placement *place, bool *negative) 
Minion of analyze_field_constant for decimal type fields. More...  
static bool  analyze_real_field_constant (THD *thd, Item_field *f, Item **const_val, Range_placement *place) 
Minion of analyze_field_constant for real type fields. More...  
static bool  analyze_year_field_constant (THD *thd, Item **const_val, Range_placement *place) 
Minion of analyze_field_constant for YEAR type fields. More...  
static bool  analyze_timestamp_field_constant (THD *thd, const Item_field *f, Item **const_val, Range_placement *place) 
Minion of analyze_field_constant for TIMESTAMP, DATETIME, DATE fields. More...  
static bool  analyze_time_field_constant (THD *thd, Item **const_val) 
static bool  analyze_field_constant (THD *thd, Item_field *f, Item **const_val, Item_func *func, bool left_has_field, Range_placement *place, bool *discount_equal, bool *negative) 
Analyze a constant's placement within (or without) the type range of the field f. More...  
static bool  fold_or_simplify (THD *thd, Item *ref_or_field, Item_func::Functype ft, bool always_true, bool manifest_result, Item **retcond, Item::cond_result *cond_value) 
We have found that the constant's value makes the predicate always true or false (modulo field nullability), so simplify or remove the condition according to this table: More...  
static bool  fold_arguments (THD *thd, Item_func *func) 
static bool  fold_arguments (THD *thd, Item_cond *cond) 
Call fold_condition on all the conditions's arguments. More...  
static Range_placement  map_less_to_greater (Range_placement place) 
Switch places of low, high, max and min. More...  
static bool  adjust_cmp_op (THD *thd, Item_func *func, bool left_has_field, bool inverse, Item *ref_or_field, Item *c, Item **retcond) 
Possibly change comparison operator because we have rounded the constant in some direction. More...  
static bool  simplify_to_equal (THD *thd, Item *ref_or_field, Item *c, Item **retcond, Item::cond_result *cond_value) 
Replace condition in retcond with "=" (i.e. Item_func_eq) and set cond_value to Item::COND_OK. More...  
bool  fold_condition (THD *thd, Item *cond, Item **retcond, Item::cond_result *cond_value, bool manifest_result) 
Fold boolean condition {=, <>, >, >=, <, <=, <=>} involving constants and fields (or references to fields), possibly determining if the condition is always true or false based on type range of the field, or possibly simplifying >=, <= to = if constant lies on the range border. More...  
enum Range_placement 
fold_condition uses analyze_field_constant to analyze constants in comparisons expressions with a view to fold the expression.
The analysis will determine the range characteristic of the constant, as represented by one value of this set. In the simple case, if the constant is within the range of the field to which it is being compared, the value used is RP_INSIDE.

static 
Possibly change comparison operator because we have rounded the constant in some direction.
Depending on the case, we change:
< to <= > to >= <= to < >= to >
[in]  thd  session thread 
[in]  func  the comparison operator 
[in]  left_has_field  true if <field> <cmp> <const>, false if inverse 
[in]  inverse  invert the logic 
[in]  ref_or_field  the field or a ref to it 
[in]  c  the constant 
[in,out]  retcond  the resulting condition 

static 
Minion of analyze_field_constant for decimal type fields.
Analyze a constant's placement within (or without) the type range of the field f. Also normalize the given constant to the type of the field if applicable.
thd  the session context  
f  the decimal typed field  
[out]  const_val  a pointer to an item tree pointer containing the constant (at execution time). May be modified if we replace or fold the constant. 
ft  the function type of the comparison  
[out]  place  the placement of the const_val relative to the range of f 
[out]  negative  true if the constant is has a (minus) sign 

static 
Analyze a constant's placement within (or without) the type range of the field f.
Also normalize the given constant to the type of the field if applicable.
thd  the session context  
f  the field  
[out]  const_val  a pointer to an item tree pointer containing the constant (at execution time). May be modified if we replace or fold the constant. 
func  the function of the comparison  
left_has_field  the field is the left operand  
[out]  place  the placement of the const_val relative to the range of f 
[out]  discount_equal  set to true: caller should replace GE with GT or LE with LT. 
[out]  negative  true if the constant (decimal) is has a (minus) sign 

static 
Minion of analyze_field_constant for integer type fields.
Analyze a constant's placement within (or without) the type range of the field f. Also normalize the given constant to the type of the field if applicable.
thd  the session context  
f  the integer typed field  
[out]  const_val  a pointer to an item tree pointer containing the constant (at execution time). May be modified if we replace or fold the constant. 
ft  the function type of the comparison  
left_has_field  the field is the left operand  
[out]  place  the placement of the const_val relative to the range of f 
[out]  discount_equal  set to true: caller should replace GE with GT or LE with LT. 

static 
Minion of analyze_field_constant for real type fields.
Analyze a constant's placement within (or without) the type range of the field f. Also normalize the given constant to the type of the field if applicable.
thd  the session context  
f  the real (FLOAT, DOUBLE) typed field  
[out]  const_val  a pointer to an item tree pointer containing the constant (at execution time). May be modified if we replace or fold the constant. 
[out]  place  the placement of the const_val relative to 

static 
Minion of analyze_field_constant for TIMESTAMP, DATETIME, DATE fields.
Analyze a constant's placement within (or without) the type range of the field f. Also normalize the given constant to the type of the field if applicable.
thd  the session context  
f  the field  
[out]  const_val  a pointer to an item tree pointer containing the constant (at execution time). May be modified if we replace or fold the constant. 
[out]  place  the placement of the const_val relative to 

static 
Minion of analyze_field_constant for YEAR type fields.
Analyze a constant's placement within (or without) the type range of the field f. Also normalize the given constant to the type of the field if applicable.
thd  the session context  
[out]  const_val  a pointer to an item tree pointer containing the constant (at execution time). May be modified if we replace or fold the constant. 
[out]  place  the placement of the const_val relative to 
Call fold_condition on all the conditions's arguments.
thd  the session state 
cond  the condition 
bool fold_condition  (  THD *  thd, 
Item *  cond,  
Item **  retcond,  
Item::cond_result *  cond_value,  
bool  manifest_result = false 

) 
Fold boolean condition {=, <>, >, >=, <, <=, <=>} involving constants and fields (or references to fields), possibly determining if the condition is always true or false based on type range of the field, or possibly simplifying >=, <= to = if constant lies on the range border.
Also fold such conditions if they are present as arguments of other functions in the same way, except there we replace the arguments with manifest FALSE or field <> field to exclude NULLs if the field is nullable, cf. ignore_unknown().
The constants are always converted to constants of the type of the field, no widening of the field type in therefore necessary after this folding for comparison to occur. When converting constants to decimal (when the field is a decimal), the constant will receive the same number of fractional decimals as the field. If decimal constant fraction truncation occurs, the comparison {GT,GE,LT,LE} logic is adjusted to preserve semantics without widening the field's type.
thd  session state  
cond  the condition to handle  
[out]  retcond  condition after const removal 
[out]  cond_value  resulting value of the condition

manifest_result  For IS NOT NULL on a not nullable item, if true, return item TRUE (1), else remove condition and return COND_TRUE. For cmp items, this is determined by Item_bool_func2::ignore_unknown. 

static 
Minion of analyze_int_field_constant.
Analyze decimal d. Convert decimal constant d to int if d has a zero fraction part and is within integer range. If it can't be converted to int, it is out of range, and we set place accordingly. If it has a nonzero fraction part, we fold. Used for = and <>.
[in]  thd  the session context 
[in,out]  const_val  the constant we are folding 
[in,out]  place  the range placement of the constant as analyzed 
[in]  f  the field the constant is being compared to 
[in]  d  the decimal value of the constant 

static 
We have found that the constant's value makes the predicate always true or false (modulo field nullability), so simplify or remove the condition according to this table:
always_true == true !top_level_item top_level_item  nullable  IF(NULL, NULL, 1)  IS NOT NULL  !nullable  TRUE (1)  COND_TRUE   always_true == false !top_level_item top_level_item  nullable  field <> field [*]  COND_FALSE  !nullable  FALSE (0)  COND_FALSE   [*] for the "<=>" operator, we fold to FALSE (0) in this case.
thd  current session context  
ref_or_field  a field (that is being being compared to a constant) or a ref to such a field.  
ft  function type  
always_true  true if the predicate is found to be always true  
manifest_result  true if we should create an item to represent the truth value (and replace the condition with it) instead of returning nullptr in *cond_value and {Item::COND_TRUE, Item::COND_FALSE} in *cond_value  
[out]  retcond  a pointer to the condition altered by this method 
[out]  cond_value  the resulting condition value 

static 
Switch places of low, high, max and min.

static 
Minion of analyze_int_field_constant.
Round up or down decimal d, then convert constant to int if possible. If d has a fraction part, set discount_equal. If it can't be converted to int, it is out of range, and we fold by setting place accordingly.
[in]  thd  the session context 
[in,out]  const_val  the constant we are folding 
[in,out]  place  the range placement of the constant as analyzed 
[in]  f  the field the constant is being compared to 
[in]  d  the decimal value of the constant 
[in]  up  true if we are to round up, else we round down 
[out]  discount_equal  set to true of d has a nonzero fraction part 

static 
Replace condition in retcond with "=" (i.e. Item_func_eq) and set cond_value to Item::COND_OK.
thd  session context  
ref_or_field  the field being compared to a constant in the retcond  
c  the constant being compared to a field in the retcond  
[in,out]  retcond  the comparison condition 
[out]  cond_value  the resulting condition value 