stream_group_accumulator.go

// 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: Andrei Matei (andreimatei1@gmail.com)

package distsqlrun

import (
	"github.com/cockroachdb/cockroach/pkg/sql/sqlbase"
	"github.com/pkg/errors"
)

// streamGroupAccumulator groups input rows coming from src into groups dictated
// by equality according to the ordering columns.
type streamGroupAccumulator struct {
	src NoMetadataRowSource
	// srcConsumed is set once src has been exhausted.
	srcConsumed bool
	ordering    sqlbase.ColumnOrdering

	// curGroup maintains the rows accumulated in the current group. The client
	// reads them with advanceGroup().
	curGroup   []sqlbase.EncDatumRow
	datumAlloc sqlbase.DatumAlloc
}

func makeStreamGroupAccumulator(
	src NoMetadataRowSource, ordering sqlbase.ColumnOrdering,
) streamGroupAccumulator {
	return streamGroupAccumulator{src: src, ordering: ordering}
}

// peekAtCurrentGroup returns the first row of the current group.
func (s *streamGroupAccumulator) peekAtCurrentGroup() (sqlbase.EncDatumRow, error) {
	// On all but the very first call, either there will be (one or all) rows
	// accumulated already in the current group, or srcConsumed will be set.
	if s.srcConsumed {
		return nil, nil
	}
	if len(s.curGroup) == 0 {
		row, err := s.src.NextRow()
		if err != nil {
			return nil, err
		}
		if row != nil {
			s.curGroup = append(s.curGroup, row)
		} else {
			s.srcConsumed = true
			return nil, nil
		}
	}
	return s.curGroup[0], nil
}

// advanceGroup returns all rows of the current group and advances the internal
// state to the next group, so that a subsequent peekAtCurrentGroup() will
// return the first row of the next group.
func (s *streamGroupAccumulator) advanceGroup() ([]sqlbase.EncDatumRow, error) {
	if s.srcConsumed {
		// If src has been exhausted, then we also must have advanced away from the
		// last group.
		return nil, nil
	}

	for {
		row, err := s.src.NextRow()
		if err != nil {
			return nil, err
		}
		if row == nil {
			s.srcConsumed = true
			return s.curGroup, nil
		}

		if len(s.curGroup) == 0 {
			s.curGroup = append(s.curGroup, row)
			continue
		}

		cmp, err := s.curGroup[0].Compare(&s.datumAlloc, s.ordering, row)
		if err != nil {
			return nil, err
		}
		if cmp == 0 {
			s.curGroup = append(s.curGroup, row)
		} else if cmp == 1 {
			return nil, errors.Errorf("detected badly ordered input: %s > %s, but expected '<'",
				s.curGroup[0], row)
		} else {
			ret := s.curGroup
			s.curGroup = []sqlbase.EncDatumRow{row}
			return ret, nil
		}
	}
}