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)}
}