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ordering.go
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ordering.go
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// Copyright 2016 The Cockroach Authors.
//
// 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.
//
// Author: Radu Berinde (radu@cockroachlabs.com)
package sqlbase
import (
"github.com/cockroachdb/cockroach/pkg/sql/parser"
"github.com/cockroachdb/cockroach/pkg/util/encoding"
)
// ColumnOrderInfo describes a column (as an index) and a desired order
// direction.
type ColumnOrderInfo struct {
ColIdx int
Direction encoding.Direction
}
// ColumnOrdering is used to describe a desired column ordering. For example,
// []ColumnOrderInfo{ {3, true}, {1, false} }
// represents an ordering first by column 3 (descending), then by column 1 (ascending).
type ColumnOrdering []ColumnOrderInfo
// IsPrefixOf returns true if the receiver ordering matches a prefix of the
// given ordering. In this case, rows with an order conforming to b
// automatically conform to a.
func (a ColumnOrdering) IsPrefixOf(b ColumnOrdering) bool {
if len(a) > len(b) {
return false
}
for i := range a {
if a[i] != b[i] {
return false
}
}
return true
}
// CompareDatums compares two datum rows according to a column ordering. Returns:
// - 0 if lhs and rhs are equal on the ordering columns;
// - less than 0 if lhs comes first;
// - greater than 0 if rhs comes first.
func CompareDatums(
ordering ColumnOrdering, evalCtx *parser.EvalContext, lhs, rhs parser.Datums,
) int {
for _, c := range ordering {
// TODO(pmattis): This is assuming that the datum types are compatible. I'm
// not sure this always holds as `CASE` expressions can return different
// types for a column for different rows. Investigate how other RDBMs
// handle this.
if cmp := lhs[c.ColIdx].Compare(evalCtx, rhs[c.ColIdx]); cmp != 0 {
if c.Direction == encoding.Descending {
cmp = -cmp
}
return cmp
}
}
return 0
}