github.com/cockroachdb/cockroach@v20.2.0-alpha.1+incompatible/pkg/sql/sqlbase/ordering.go (about)

     1  // Copyright 2016 The Cockroach Authors.
     2  //
     3  // Use of this software is governed by the Business Source License
     4  // included in the file licenses/BSL.txt.
     5  //
     6  // As of the Change Date specified in that file, in accordance with
     7  // the Business Source License, use of this software will be governed
     8  // by the Apache License, Version 2.0, included in the file
     9  // licenses/APL.txt.
    10  
    11  package sqlbase
    12  
    13  import (
    14  	"github.com/cockroachdb/cockroach/pkg/sql/sem/tree"
    15  	"github.com/cockroachdb/cockroach/pkg/util/encoding"
    16  )
    17  
    18  // ColumnOrderInfo describes a column (as an index) and a desired order
    19  // direction.
    20  type ColumnOrderInfo struct {
    21  	ColIdx    int
    22  	Direction encoding.Direction
    23  }
    24  
    25  // ColumnOrdering is used to describe a desired column ordering. For example,
    26  //     []ColumnOrderInfo{ {3, encoding.Descending}, {1, encoding.Ascending} }
    27  // represents an ordering first by column 3 (descending), then by column 1 (ascending).
    28  type ColumnOrdering []ColumnOrderInfo
    29  
    30  // NoOrdering is used to indicate an empty ColumnOrdering.
    31  var NoOrdering ColumnOrdering
    32  
    33  // CompareDatums compares two datum rows according to a column ordering. Returns:
    34  //  - 0 if lhs and rhs are equal on the ordering columns;
    35  //  - less than 0 if lhs comes first;
    36  //  - greater than 0 if rhs comes first.
    37  func CompareDatums(ordering ColumnOrdering, evalCtx *tree.EvalContext, lhs, rhs tree.Datums) int {
    38  	for _, c := range ordering {
    39  		// TODO(pmattis): This is assuming that the datum types are compatible. I'm
    40  		// not sure this always holds as `CASE` expressions can return different
    41  		// types for a column for different rows. Investigate how other RDBMs
    42  		// handle this.
    43  		if cmp := lhs[c.ColIdx].Compare(evalCtx, rhs[c.ColIdx]); cmp != 0 {
    44  			if c.Direction == encoding.Descending {
    45  				cmp = -cmp
    46  			}
    47  			return cmp
    48  		}
    49  	}
    50  	return 0
    51  }