74 lines
2.0 KiB
Go
74 lines
2.0 KiB
Go
// Copyright 2015 PingCAP, Inc.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// See the License for the specific language governing permissions and
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// limitations under the License.
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package expression
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import (
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"fmt"
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"github.com/juju/errors"
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"github.com/pingcap/tidb/context"
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)
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var (
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_ Expression = (*Position)(nil)
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)
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// Position is the expression for order by and group by position.
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// MySQL use position expression started from 1, it looks a little confused inner.
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// maybe later we will use 0 at first.
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type Position struct {
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// N is the position, started from 1 now.
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N int
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// Name is the corresponding field name if we want better format and explain instead of position.
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Name string
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}
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// Clone implements the Expression Clone interface.
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func (p *Position) Clone() Expression {
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newP := *p
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return &newP
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}
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// IsStatic implements the Expression IsStatic interface, always returns false.
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func (p *Position) IsStatic() bool {
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return false
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}
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// String implements the Expression String interface.
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func (p *Position) String() string {
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if len(p.Name) > 0 {
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return p.Name
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}
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return fmt.Sprintf("%d", p.N)
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}
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// Eval implements the Expression Eval interface.
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func (p *Position) Eval(ctx context.Context, args map[interface{}]interface{}) (v interface{}, err error) {
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f, ok := args[ExprEvalPositionFunc]
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if !ok {
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return nil, errors.Errorf("no eval position function")
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}
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got, ok := f.(func(int) (interface{}, error))
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if !ok {
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return nil, errors.Errorf("invalid eval position function format")
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}
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return got(p.N)
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}
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// Accept implements Expression Accept interface.
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func (p *Position) Accept(v Visitor) (Expression, error) {
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return v.VisitPosition(p)
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}
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