executor: union scan refactor, optimize the membuffer decoding process (#45051)

ref pingcap/tidb#43249

executor/builder.go

@@ -5115,6 +5115,13 @@ func NewRowDecoder(ctx sessionctx.Context, schema *expression.Schema, tbl *model
 		}
 	}
 	defVal := func(i int, chk *chunk.Chunk) error {
+		if reqCols[i].ID < 0 {
+			// model.ExtraHandleID, ExtraPidColID, ExtraPhysTblID... etc
+			// Don't set the default value for that column.
+			chk.AppendNull(i)
+			return nil
+		}
+
 		ci := getColInfoByID(tbl, reqCols[i].ID)
 		d, err := table.GetColOriginDefaultValue(ctx, ci)
 		if err != nil {

executor/mem_reader.go

@@ -199,6 +199,7 @@ type allocBuf struct {
 	// cache for decode handle.
 	handleBytes []byte
 	rd          *rowcodec.BytesDecoder
+	cd          *rowcodec.ChunkDecoder
 }
 
 func buildMemTableReader(ctx context.Context, us *UnionScanExec, kvRanges []kv.KeyRange) *memTableReader {
@@ -230,6 +231,7 @@ func buildMemTableReader(ctx context.Context, us *UnionScanExec, kvRanges []kv.K
 		}
 		return tablecodec.EncodeValue(us.Ctx().GetSessionVars().StmtCtx, nil, d)
 	}
+	cd := NewRowDecoder(us.Ctx(), us.Schema(), us.table.Meta())
 	rd := rowcodec.NewByteDecoder(colInfo, pkColIDs, defVal, us.Ctx().GetSessionVars().Location())
 	return &memTableReader{
 		ctx:           us.Ctx(),
@@ -242,6 +244,7 @@ func buildMemTableReader(ctx context.Context, us *UnionScanExec, kvRanges []kv.K
 		buffer: allocBuf{
 			handleBytes: make([]byte, 0, 16),
 			rd:          rd,
+			cd:          cd,
 		},
 		pkColIDs:    pkColIDs,
 		cacheTable:  us.cacheTable,
@@ -255,6 +258,10 @@ type txnMemBufferIter struct {
 	txn  kv.Transaction
 	idx  int
 	curr kv.Iterator
+
+	cd       *rowcodec.ChunkDecoder
+	chk      *chunk.Chunk
+	datumRow []types.Datum
 }
 
 func (iter *txnMemBufferIter) Next() ([]types.Datum, error) {
@@ -299,22 +306,47 @@ func (iter *txnMemBufferIter) next() ([]types.Datum, error) {
 			continue
 		}
 
-		mutableRow := chunk.MutRowFromTypes(iter.retFieldTypes)
-		resultRows := make([]types.Datum, len(iter.columns))
-		resultRows, err = iter.decodeRecordKeyValue(curr.Key(), curr.Value(), &resultRows)
+		handle, err := tablecodec.DecodeRowKey(curr.Key())
+		if err != nil {
+			return nil, errors.Trace(err)
+		}
+		iter.chk.Reset()
+
+		if !rowcodec.IsNewFormat(curr.Value()) {
+			// TODO: remove the legacy code!
+			// fallback to the old way.
+			iter.datumRow, err = iter.decodeRecordKeyValue(curr.Key(), curr.Value(), &iter.datumRow)
+			if err != nil {
+				return nil, errors.Trace(err)
+			}
+
+			mutableRow := chunk.MutRowFromTypes(iter.retFieldTypes)
+			mutableRow.SetDatums(iter.datumRow...)
+			matched, _, err := expression.EvalBool(iter.ctx, iter.conditions, mutableRow.ToRow())
+			if err != nil {
+				return nil, errors.Trace(err)
+			}
+			if !matched {
+				continue
+			}
+			return iter.datumRow, curr.Next()
+		}
+
+		err = iter.cd.DecodeToChunk(curr.Value(), handle, iter.chk)
 		if err != nil {
 			return nil, errors.Trace(err)
 		}
 
-		mutableRow.SetDatums(resultRows...)
-		matched, _, err := expression.EvalBool(iter.ctx, iter.conditions, mutableRow.ToRow())
+		row := iter.chk.GetRow(0)
+		matched, _, err := expression.EvalBool(iter.ctx, iter.conditions, row)
 		if err != nil {
 			return nil, errors.Trace(err)
 		}
 		if !matched {
 			continue
 		}
-		return resultRows, curr.Next()
+		ret := row.GetDatumRowWithBuffer(iter.retFieldTypes, iter.datumRow)
+		return ret, curr.Next()
 	}
 	return nil, err
 }
@@ -341,6 +373,9 @@ func (m *memTableReader) getMemRowsIter(ctx context.Context) (memRowsIter, error
 	return &txnMemBufferIter{
 		memTableReader: m,
 		txn:            txn,
+		cd:             m.buffer.cd,
+		chk:            chunk.New(m.retFieldTypes, 1, 1),
+		datumRow:       make([]types.Datum, len(m.retFieldTypes)),
 	}, nil
 }
 

table/column.go

@@ -629,7 +629,7 @@ func getColDefaultValueFromNil(ctx sessionctx.Context, col *model.ColumnInfo, ar
 		sc.AppendWarning(ErrColumnCantNull.FastGenByArgs(col.Name))
 		return GetZeroValue(col), nil
 	}
-	return types.Datum{}, ErrNoDefaultValue.FastGenByArgs(col.Name)
+	return types.Datum{}, ErrNoDefaultValue.GenWithStackByArgs(col.Name)
 }
 
 // GetZeroValue gets zero value for given column type.