tidb first_row 源码
tidb first_row 代码
文件路径：/expression/aggregation/first_row.go
// Copyright 2017 PingCAP, 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 aggregation
import (
"github.com/pingcap/errors"
"github.com/pingcap/tidb/sessionctx/stmtctx"
"github.com/pingcap/tidb/types"
"github.com/pingcap/tidb/util/chunk"
)
type firstRowFunction struct {
aggFunction
}
// Update implements Aggregation interface.
func (ff *firstRowFunction) Update(evalCtx *AggEvaluateContext, sc *stmtctx.StatementContext, row chunk.Row) error {
if evalCtx.GotFirstRow {
return nil
}
if len(ff.Args) != 1 {
return errors.New("Wrong number of args for AggFuncFirstRow")
}
value, err := ff.Args[0].Eval(row)
if err != nil {
return err
}
value.Copy(&evalCtx.Value)
evalCtx.GotFirstRow = true
return nil
}
// GetResult implements Aggregation interface.
func (ff *firstRowFunction) GetResult(evalCtx *AggEvaluateContext) types.Datum {
return evalCtx.Value
}
func (ff *firstRowFunction) ResetContext(_ *stmtctx.StatementContext, evalCtx *AggEvaluateContext) {
evalCtx.GotFirstRow = false
}
// GetPartialResult implements Aggregation interface.
func (ff *firstRowFunction) GetPartialResult(evalCtx *AggEvaluateContext) []types.Datum {
return []types.Datum{ff.GetResult(evalCtx)}
}
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