github.com/hasnat/dolt/go@v0.0.0-20210628190320-9eb5d843fbb7/libraries/doltcore/table/composite_table_reader.go (about)

     1  // Copyright 2020 Dolthub, Inc.
     2  //
     3  // Licensed under the Apache License, Version 2.0 (the "License");
     4  // you may not use this file except in compliance with the License.
     5  // You may obtain a copy of the License at
     6  //
     7  //     http://www.apache.org/licenses/LICENSE-2.0
     8  //
     9  // Unless required by applicable law or agreed to in writing, software
    10  // distributed under the License is distributed on an "AS IS" BASIS,
    11  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    12  // See the License for the specific language governing permissions and
    13  // limitations under the License.
    14  
    15  package table
    16  
    17  import (
    18  	"context"
    19  	"io"
    20  
    21  	"github.com/dolthub/dolt/go/libraries/doltcore/row"
    22  	"github.com/dolthub/dolt/go/libraries/doltcore/schema"
    23  )
    24  
    25  // CompositeTableReader is a TableReader implementation which will concatenate the results
    26  // of multiple TableReader instances into a single set of results.
    27  type CompositeTableReader struct {
    28  	sch     schema.Schema
    29  	readers []TableReadCloser
    30  	idx     int
    31  }
    32  
    33  // NewCompositeTableReader creates a new CompositeTableReader instance from a slice of TableReadClosers.
    34  func NewCompositeTableReader(readers []TableReadCloser) (*CompositeTableReader, error) {
    35  	if len(readers) == 0 {
    36  		panic("nothing to iterate")
    37  	}
    38  
    39  	sch := readers[0].GetSchema()
    40  	for i := 1; i < len(readers); i++ {
    41  		otherSch := readers[i].GetSchema()
    42  		if !schema.SchemasAreEqual(sch, otherSch) {
    43  			panic("readers must have the same schema")
    44  		}
    45  	}
    46  
    47  	return &CompositeTableReader{sch: sch, readers: readers, idx: 0}, nil
    48  }
    49  
    50  // GetSchema gets the schema of the rows that this reader will return
    51  func (rd *CompositeTableReader) GetSchema() schema.Schema {
    52  	return rd.sch
    53  }
    54  
    55  // ReadRow reads a row from a table.  If there is a bad row the returned error will be non nil, and calling
    56  // IsBadRow(err) will be return true. This is a potentially non-fatal error and callers can decide if they want to
    57  // continue on a bad row, or fail.
    58  func (rd *CompositeTableReader) ReadRow(ctx context.Context) (row.Row, error) {
    59  	for rd.idx < len(rd.readers) {
    60  		r, err := rd.readers[rd.idx].ReadRow(ctx)
    61  
    62  		if err == io.EOF {
    63  			rd.idx++
    64  			continue
    65  		} else if err != nil {
    66  			return nil, err
    67  		}
    68  
    69  		return r, nil
    70  	}
    71  
    72  	return nil, io.EOF
    73  }
    74  
    75  // VerifySchema checks that the incoming schema matches the schema from the existing table
    76  func (rd *CompositeTableReader) VerifySchema(outSch schema.Schema) (bool, error) {
    77  	return schema.VerifyInSchema(rd.sch, outSch)
    78  }
    79  
    80  // Close should release resources being held
    81  func (rd *CompositeTableReader) Close(ctx context.Context) error {
    82  	var firstErr error
    83  	for _, rdr := range rd.readers {
    84  		err := rdr.Close(ctx)
    85  
    86  		if firstErr == nil {
    87  			firstErr = err
    88  		}
    89  	}
    90  
    91  	return firstErr
    92  }