github.com/dolthub/go-mysql-server@v0.18.0/sql/coercibility.go (about)

     1  // Copyright 2023 Dolthub, Inc.
     2  //
     3  // Licensed under the Apache License, Version 2.0 (the "License");
     4  // you may not use this file except in compliance with the License.
     5  // You may obtain a copy of the License at
     6  //
     7  //     http://www.apache.org/licenses/LICENSE-2.0
     8  //
     9  // Unless required by applicable law or agreed to in writing, software
    10  // distributed under the License is distributed on an "AS IS" BASIS,
    11  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    12  // See the License for the specific language governing permissions and
    13  // limitations under the License.
    14  
    15  package sql
    16  
    17  import "strings"
    18  
    19  // CollationCoercible represents the coercibility of an expression or node. Although the resulting value from the node
    20  // or expression may be NULL, this interface returns the coercibility as though a NULL would not be returned.
    21  type CollationCoercible interface {
    22  	// CollationCoercibility returns the collation and coercibility of the expression or node.
    23  	CollationCoercibility(ctx *Context) (collation CollationID, coercibility byte)
    24  }
    25  
    26  // ResolveCoercibility returns the collation to use by comparing coercibility, along with giving priority to binary
    27  // collations. This is an approximation of MySQL's coercibility rules:
    28  // https://dev.mysql.com/doc/refman/8.0/en/charset-collation-coercibility.html
    29  // As we do not implement the full coercion pipeline, we make some assumptions when information is missing, and never
    30  // error even when MySQL would find an expression invalid.
    31  func ResolveCoercibility(leftCollation CollationID, leftCoercibility byte, rightCollation CollationID, rightCoercibility byte) (CollationID, byte) {
    32  	if leftCoercibility < rightCoercibility {
    33  		return leftCollation, leftCoercibility
    34  	} else if leftCoercibility > rightCoercibility {
    35  		return rightCollation, rightCoercibility
    36  	} else if leftCollation == rightCollation {
    37  		return leftCollation, leftCoercibility
    38  	} else if leftCollation == Collation_Unspecified {
    39  		return rightCollation, rightCoercibility
    40  	} else if rightCollation == Collation_Unspecified {
    41  		return leftCollation, leftCoercibility
    42  	} else { // Collations are not equal
    43  		leftCharset := leftCollation.CharacterSet()
    44  		rightCharset := rightCollation.CharacterSet()
    45  		if leftCharset != rightCharset {
    46  			if leftCharset.MaxLength() == 1 && rightCharset.MaxLength() > 1 { // Left non-Unicode, Right Unicode
    47  				return rightCollation, rightCoercibility
    48  			} else if leftCharset.MaxLength() > 1 && rightCharset.MaxLength() == 1 { // Left Unicode, Right non-Unicode
    49  				return leftCollation, leftCoercibility
    50  			} else {
    51  				return Collation_binary, 7
    52  			}
    53  		} else { // Character sets are equal
    54  			// If the right collation is not _bin, then we default to the left collation (regardless of whether it is
    55  			// or is not _bin).
    56  			if strings.HasSuffix(rightCollation.Name(), "_bin") {
    57  				return rightCollation, rightCoercibility
    58  			} else {
    59  				return leftCollation, leftCoercibility
    60  			}
    61  		}
    62  	}
    63  }
    64  
    65  // GetCoercibility returns the coercibility of the given node or expression.
    66  func GetCoercibility(ctx *Context, nodeOrExpr interface{}) (collation CollationID, coercibility byte) {
    67  	if nodeOrExpr == nil {
    68  		return Collation_binary, 6
    69  	}
    70  	if cc, ok := nodeOrExpr.(CollationCoercible); ok {
    71  		return cc.CollationCoercibility(ctx)
    72  	}
    73  	collation = Collation_binary
    74  	coercibility = 7
    75  	// We check for Node, Expressioner, and Expression and take the lowest coercibility since CollationCoercible was
    76  	// not explicitly implemented
    77  	if n, ok := nodeOrExpr.(Node); ok {
    78  		for _, child := range n.Children() {
    79  			nextCollation, nextCoercibility := GetCoercibility(ctx, child)
    80  			collation, coercibility = ResolveCoercibility(collation, coercibility, nextCollation, nextCoercibility)
    81  		}
    82  	}
    83  	if e, ok := nodeOrExpr.(Expressioner); ok {
    84  		for _, child := range e.Expressions() {
    85  			nextCollation, nextCoercibility := GetCoercibility(ctx, child)
    86  			collation, coercibility = ResolveCoercibility(collation, coercibility, nextCollation, nextCoercibility)
    87  		}
    88  	}
    89  	if e, ok := nodeOrExpr.(Expression); ok {
    90  		for _, child := range e.Children() {
    91  			nextCollation, nextCoercibility := GetCoercibility(ctx, child)
    92  			collation, coercibility = ResolveCoercibility(collation, coercibility, nextCollation, nextCoercibility)
    93  		}
    94  	}
    95  	return collation, coercibility
    96  }