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columndefault.go
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columndefault.go
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// Copyright 2020-2021 Dolthub, Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package sql
import (
"fmt"
)
// ColumnDefaultValue is an expression representing the default value of a column. May represent both a default literal
// and a default expression. A nil pointer of this type represents an implicit default value and is thus valid, so all
// method calls will return without error.
type ColumnDefaultValue struct {
Expression // the expression representing this default value
outType Type // if non-nil, converts the output of the expression into this type
literal bool // whether the default value is a literal or expression
returnNil bool // if the expression returns a nil value, then this determines whether the result is returned or an error is returned
}
var _ Expression = (*ColumnDefaultValue)(nil)
// NewColumnDefaultValue returns a new ColumnDefaultValue expression.
func NewColumnDefaultValue(expr Expression, outType Type, representsLiteral bool, mayReturnNil bool) (*ColumnDefaultValue, error) {
colDefault := &ColumnDefaultValue{
Expression: expr,
outType: outType,
literal: representsLiteral,
returnNil: mayReturnNil,
}
if err := colDefault.checkType(outType); err != nil {
return nil, err
}
return colDefault, nil
}
// Children implements sql.Expression
func (e *ColumnDefaultValue) Children() []Expression {
if e == nil {
return nil
}
return []Expression{e.Expression}
}
// Eval implements sql.Expression
func (e *ColumnDefaultValue) Eval(ctx *Context, r Row) (interface{}, error) {
if e == nil {
return nil, nil
}
val, err := e.Expression.Eval(ctx, r)
if err != nil {
return nil, err
}
if e.outType != nil {
val, err = e.outType.Convert(val)
if err != nil {
return nil, err
}
}
if val == nil && !e.returnNil {
return nil, ErrColumnDefaultReturnedNull.New()
}
return val, nil
}
// IsLiteral returns whether this expression represents a literal default value (otherwise it's an expression default value).
func (e *ColumnDefaultValue) IsLiteral() bool {
if e == nil {
return true // we return the literal nil, hence true
}
return e.literal
}
// IsNullable implements sql.Expression
func (e *ColumnDefaultValue) IsNullable() bool {
if e == nil {
return true
}
if !e.returnNil {
return false
}
return e.Expression.IsNullable()
}
// Resolved implements sql.Expression
func (e *ColumnDefaultValue) Resolved() bool {
if e == nil {
return true
}
if e.outType == nil {
return false
}
return e.Expression.Resolved()
}
// String implements sql.Expression
func (e *ColumnDefaultValue) String() string {
//TODO: currently (2+2)/2 will, when output as a string, give (2 + 2 / 2), which is clearly wrong
if e == nil {
return ""
}
// https://dev.mysql.com/doc/refman/8.0/en/data-type-defaults.html
// The default value specified in a DEFAULT clause can be a literal constant or an expression. With one exception,
// enclose expression default values within parentheses to distinguish them from literal constant default values.
if e.literal {
return e.Expression.String()
} else {
return fmt.Sprintf("(%s)", e.Expression.String())
}
}
// Type implements sql.Expression
func (e *ColumnDefaultValue) Type() Type {
if e == nil {
return Null
}
if e.outType == nil {
return e.Expression.Type()
}
return e.outType
}
// WithChildren implements sql.Expression
func (e *ColumnDefaultValue) WithChildren(ctx *Context, children ...Expression) (Expression, error) {
if len(children) != 1 {
return nil, ErrInvalidChildrenNumber.New(e, len(children), 1)
}
if e == nil {
return NewColumnDefaultValue(children[0], e.outType, len(children[0].Children()) == 0, true) //impossible to know, best guess
} else {
return NewColumnDefaultValue(children[0], e.outType, e.literal, e.returnNil)
}
}
// WithType returns a new default value that converts all resulting values from the internal expression into the given type.
// If the internal expression results in a value that cannot be converted to the given type, then an error is returned.
func (e *ColumnDefaultValue) WithType(outType Type) (*ColumnDefaultValue, error) {
if e == nil {
return nil, nil
}
if err := e.checkType(outType); err != nil {
return nil, err
}
return NewColumnDefaultValue(e.Expression, outType, e.literal, e.returnNil)
}
func (e *ColumnDefaultValue) checkType(outType Type) error {
if outType != nil && e.literal {
val, err := e.Expression.Eval(NewEmptyContext(), nil) // since it's a literal, we can use an empty context
if err != nil {
return err
}
if val == nil && !e.returnNil {
return ErrIncompatibleDefaultType.New()
}
_, err = outType.Convert(val)
if err != nil {
return ErrIncompatibleDefaultType.New()
}
}
return nil
}