diff --git a/pkg/sql/distsqlrun/algebraic_set_op.go b/pkg/sql/distsqlrun/algebraic_set_op.go
index c05bf703ae60..c4c425b6e0f6 100644
--- a/pkg/sql/distsqlrun/algebraic_set_op.go
+++ b/pkg/sql/distsqlrun/algebraic_set_op.go
@@ -18,6 +18,7 @@ import (
 	"fmt"
 	"sync"
 
+	"github.com/cockroachdb/cockroach/pkg/sql/sem/tree"
 	"github.com/cockroachdb/cockroach/pkg/sql/sqlbase"
 	"github.com/cockroachdb/cockroach/pkg/util/log"
 	"github.com/cockroachdb/cockroach/pkg/util/tracing"
@@ -30,6 +31,9 @@ import (
 type algebraicSetOp struct {
 	processorBase
 
+	flowCtx *FlowCtx
+	evalCtx *tree.EvalContext
+
 	leftSource, rightSource RowSource
 	opType                  AlgebraicSetOpSpec_SetOpType
 	ordering                Ordering
@@ -47,6 +51,7 @@ func newAlgebraicSetOp(
 	output RowReceiver,
 ) (*algebraicSetOp, error) {
 	e := &algebraicSetOp{
+		flowCtx:     flowCtx,
 		leftSource:  leftSource,
 		rightSource: rightSource,
 		ordering:    spec.Ordering,
@@ -99,6 +104,8 @@ func (e *algebraicSetOp) Run(ctx context.Context, wg *sync.WaitGroup) {
 	defer e.leftSource.ConsumerDone()
 	defer e.rightSource.ConsumerDone()
 
+	e.evalCtx = e.flowCtx.NewEvalCtx()
+
 	switch e.opType {
 	case AlgebraicSetOpSpec_Except_all:
 		err := e.exceptAll(ctx)
@@ -122,11 +129,11 @@ func (e *algebraicSetOp) exceptAll(ctx context.Context) error {
 		convertToColumnOrdering(e.ordering),
 	)
 
-	leftRows, err := leftGroup.advanceGroup()
+	leftRows, err := leftGroup.advanceGroup(e.evalCtx)
 	if err != nil {
 		return err
 	}
-	rightRows, err := rightGroup.advanceGroup()
+	rightRows, err := rightGroup.advanceGroup(e.evalCtx)
 	if err != nil {
 		return err
 	}
@@ -160,6 +167,7 @@ func (e *algebraicSetOp) exceptAll(ctx context.Context) error {
 			convertToColumnOrdering(e.ordering), convertToColumnOrdering(e.ordering),
 			false, /* nullEquality */
 			e.datumAlloc,
+			e.evalCtx,
 		)
 		if err != nil {
 			return err
@@ -195,11 +203,11 @@ func (e *algebraicSetOp) exceptAll(ctx context.Context) error {
 					}
 				}
 			}
-			leftRows, err = leftGroup.advanceGroup()
+			leftRows, err = leftGroup.advanceGroup(e.evalCtx)
 			if err != nil {
 				return err
 			}
-			rightRows, err = rightGroup.advanceGroup()
+			rightRows, err = rightGroup.advanceGroup(e.evalCtx)
 			if err != nil {
 				return err
 			}
@@ -214,13 +222,13 @@ func (e *algebraicSetOp) exceptAll(ctx context.Context) error {
 					return err
 				}
 			}
-			leftRows, err = leftGroup.advanceGroup()
+			leftRows, err = leftGroup.advanceGroup(e.evalCtx)
 			if err != nil {
 				return err
 			}
 		}
 		if cmp > 0 {
-			rightRows, err = rightGroup.advanceGroup()
+			rightRows, err = rightGroup.advanceGroup(e.evalCtx)
 			if len(rightRows) == 0 {
 				break
 			}
@@ -244,7 +252,7 @@ func (e *algebraicSetOp) exceptAll(ctx context.Context) error {
 
 		// Emit all remaining rows.
 		for {
-			leftRows, err = leftGroup.advanceGroup()
+			leftRows, err = leftGroup.advanceGroup(e.evalCtx)
 			// Emit all left rows until completion/error.
 			if err != nil || len(leftRows) == 0 {
 				return err
diff --git a/pkg/sql/distsqlrun/mergejoiner.go b/pkg/sql/distsqlrun/mergejoiner.go
index fdcffb2c09c9..12fc0045367f 100644
--- a/pkg/sql/distsqlrun/mergejoiner.go
+++ b/pkg/sql/distsqlrun/mergejoiner.go
@@ -20,6 +20,7 @@ import (
 
 	"golang.org/x/net/context"
 
+	"github.com/cockroachdb/cockroach/pkg/sql/sem/tree"
 	"github.com/cockroachdb/cockroach/pkg/sql/sqlbase"
 	"github.com/cockroachdb/cockroach/pkg/util/log"
 	"github.com/cockroachdb/cockroach/pkg/util/tracing"
@@ -32,6 +33,9 @@ import (
 type mergeJoiner struct {
 	joinerBase
 
+	flowCtx *FlowCtx
+	evalCtx *tree.EvalContext
+
 	leftSource, rightSource RowSource
 
 	streamMerger streamMerger
@@ -54,6 +58,7 @@ func newMergeJoiner(
 	}
 
 	m := &mergeJoiner{
+		flowCtx:     flowCtx,
 		leftSource:  leftSource,
 		rightSource: rightSource,
 	}
@@ -90,6 +95,7 @@ func (m *mergeJoiner) Run(ctx context.Context, wg *sync.WaitGroup) {
 	log.VEventf(ctx, 2, "starting merge joiner run")
 
 	cancelChecker := sqlbase.NewCancelChecker(ctx)
+	m.evalCtx = m.flowCtx.NewEvalCtx()
 
 	for {
 		moreBatches, err := m.outputBatch(ctx, cancelChecker)
@@ -112,7 +118,7 @@ func (m *mergeJoiner) Run(ctx context.Context, wg *sync.WaitGroup) {
 func (m *mergeJoiner) outputBatch(
 	ctx context.Context, cancelChecker *sqlbase.CancelChecker,
 ) (bool, error) {
-	leftRows, rightRows, err := m.streamMerger.NextBatch()
+	leftRows, rightRows, err := m.streamMerger.NextBatch(m.evalCtx)
 	if err != nil {
 		return false, err
 	}
diff --git a/pkg/sql/distsqlrun/stream_group_accumulator.go b/pkg/sql/distsqlrun/stream_group_accumulator.go
index 24b025d82e2e..41db0c965287 100644
--- a/pkg/sql/distsqlrun/stream_group_accumulator.go
+++ b/pkg/sql/distsqlrun/stream_group_accumulator.go
@@ -71,14 +71,14 @@ func (s *streamGroupAccumulator) peekAtCurrentGroup() (sqlbase.EncDatumRow, erro
 // 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) {
+func (s *streamGroupAccumulator) advanceGroup(
+	evalCtx *tree.EvalContext,
+) ([]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
 	}
-	// TODO(radu): plumb EvalContext
-	evalCtx := &tree.EvalContext{}
 
 	for {
 		row, err := s.src.NextRow()
@@ -91,6 +91,9 @@ func (s *streamGroupAccumulator) advanceGroup() ([]sqlbase.EncDatumRow, error) {
 		}
 
 		if len(s.curGroup) == 0 {
+			if s.curGroup == nil {
+				s.curGroup = make([]sqlbase.EncDatumRow, 0, 64)
+			}
 			s.curGroup = append(s.curGroup, row)
 			continue
 		}
@@ -107,8 +110,15 @@ func (s *streamGroupAccumulator) advanceGroup() ([]sqlbase.EncDatumRow, error) {
 				s.curGroup[0].String(s.types), row.String(s.types),
 			)
 		} else {
-			ret := s.curGroup
-			s.curGroup = []sqlbase.EncDatumRow{row}
+			n := len(s.curGroup)
+			ret := s.curGroup[:n:n]
+			// The curGroup slice possibly has additional space at the end of it. Use
+			// it if possible to avoid an allocation.
+			s.curGroup = s.curGroup[n:]
+			if cap(s.curGroup) == 0 {
+				s.curGroup = make([]sqlbase.EncDatumRow, 0, 64)
+			}
+			s.curGroup = append(s.curGroup, row)
 			return ret, nil
 		}
 	}
diff --git a/pkg/sql/distsqlrun/stream_merger.go b/pkg/sql/distsqlrun/stream_merger.go
index b49a5a615b8b..0dbad59e3eff 100644
--- a/pkg/sql/distsqlrun/stream_merger.go
+++ b/pkg/sql/distsqlrun/stream_merger.go
@@ -38,7 +38,9 @@ type streamMerger struct {
 // NextBatch returns a set of rows from the left stream and a set of rows from
 // the right stream, all matching on the equality columns. One of the sets can
 // be empty.
-func (sm *streamMerger) NextBatch() ([]sqlbase.EncDatumRow, []sqlbase.EncDatumRow, error) {
+func (sm *streamMerger) NextBatch(
+	evalCtx *tree.EvalContext,
+) ([]sqlbase.EncDatumRow, []sqlbase.EncDatumRow, error) {
 	lrow, err := sm.left.peekAtCurrentGroup()
 	if err != nil {
 		return nil, nil, err
@@ -52,20 +54,21 @@ func (sm *streamMerger) NextBatch() ([]sqlbase.EncDatumRow, []sqlbase.EncDatumRo
 	}
 
 	cmp, err := CompareEncDatumRowForMerge(
-		sm.left.types, lrow, rrow, sm.left.ordering, sm.right.ordering, sm.nullEquality, &sm.datumAlloc,
+		sm.left.types, lrow, rrow, sm.left.ordering, sm.right.ordering,
+		sm.nullEquality, &sm.datumAlloc, evalCtx,
 	)
 	if err != nil {
 		return nil, nil, err
 	}
 	var leftGroup, rightGroup []sqlbase.EncDatumRow
 	if cmp <= 0 {
-		leftGroup, err = sm.left.advanceGroup()
+		leftGroup, err = sm.left.advanceGroup(evalCtx)
 		if err != nil {
 			return nil, nil, err
 		}
 	}
 	if cmp >= 0 {
-		rightGroup, err = sm.right.advanceGroup()
+		rightGroup, err = sm.right.advanceGroup(evalCtx)
 		if err != nil {
 			return nil, nil, err
 		}
@@ -89,6 +92,7 @@ func CompareEncDatumRowForMerge(
 	leftOrdering, rightOrdering sqlbase.ColumnOrdering,
 	nullEquality bool,
 	da *sqlbase.DatumAlloc,
+	evalCtx *tree.EvalContext,
 ) (int, error) {
 	if lhs == nil && rhs == nil {
 		return 0, nil
@@ -105,9 +109,6 @@ func CompareEncDatumRowForMerge(
 		)
 	}
 
-	// TODO(radu): plumb EvalContext
-	evalCtx := &tree.EvalContext{}
-
 	for i, ord := range leftOrdering {
 		lIdx := ord.ColIdx
 		rIdx := rightOrdering[i].ColIdx