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

     1  // Copyright 2019 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 colexec
    12  
    13  import (
    14  	"context"
    15  
    16  	"github.com/cockroachdb/cockroach/pkg/col/coldata"
    17  	"github.com/cockroachdb/cockroach/pkg/sql/colexecbase"
    18  )
    19  
    20  // bufferOp is an operator that buffers a single batch at a time from an input,
    21  // and makes it available to be read multiple times by downstream consumers.
    22  type bufferOp struct {
    23  	OneInputNode
    24  	initStatus OperatorInitStatus
    25  
    26  	// read is true if someone has read the current batch already.
    27  	read  bool
    28  	batch coldata.Batch
    29  }
    30  
    31  var _ colexecbase.Operator = &bufferOp{}
    32  
    33  // NewBufferOp returns a new bufferOp, initialized to buffer batches from the
    34  // supplied input.
    35  func NewBufferOp(input colexecbase.Operator) colexecbase.Operator {
    36  	return &bufferOp{
    37  		OneInputNode: NewOneInputNode(input),
    38  	}
    39  }
    40  
    41  func (b *bufferOp) Init() {
    42  	// bufferOp can be an input to multiple operator chains, so Init on it can be
    43  	// called multiple times. However, we do not want to call Init many times on
    44  	// the input to bufferOp, so we do this check whether Init has already been
    45  	// performed.
    46  	if b.initStatus == OperatorNotInitialized {
    47  		b.input.Init()
    48  		b.initStatus = OperatorInitialized
    49  	}
    50  }
    51  
    52  // rewind resets this buffer to be readable again.
    53  // NOTE: it is the caller responsibility to restore the batch into the desired
    54  // state.
    55  func (b *bufferOp) rewind() {
    56  	b.read = false
    57  }
    58  
    59  // advance reads the next batch from the input into the buffer, preparing itself
    60  // for reads.
    61  func (b *bufferOp) advance(ctx context.Context) {
    62  	b.batch = b.input.Next(ctx)
    63  	b.rewind()
    64  }
    65  
    66  func (b *bufferOp) Next(ctx context.Context) coldata.Batch {
    67  	if b.read {
    68  		return coldata.ZeroBatch
    69  	}
    70  	b.read = true
    71  	return b.batch
    72  }