db: add initMergingIterLevel to flushable interface

batch.go

@@ -2043,6 +2043,14 @@ func (b *flushableBatch) Swap(i, j int) {
 	b.offsets[i], b.offsets[j] = b.offsets[j], b.offsets[i]
 }
 
+// initMergingIterLevel is part of the flushable interface.
+func (b *flushableBatch) initMergingIterLevel(
+	ctx context.Context, o *IterOptions, mil *mergingIterLevel,
+) {
+	mil.iter = b.newIter(o)
+	mil.rangeDelIter = b.newRangeDelIter(o)
+}
+
 // newIter is part of the flushable interface.
 func (b *flushableBatch) newIter(o *IterOptions) internalIterator {
 	return &flushableBatchIter{

db.go

@@ -1451,10 +1451,8 @@ func (i *Iterator) constructPointIter(
 		// Next are the memtables.
 		for j := len(memtables) - 1; j >= 0; j-- {
 			mem := memtables[j]
-			mlevels = append(mlevels, mergingIterLevel{
-				iter:         mem.newIter(&i.opts),
-				rangeDelIter: mem.newRangeDelIter(&i.opts),
-			})
+			mlevels = append(mlevels, mergingIterLevel{})
+			mem.initMergingIterLevel(ctx, &i.opts, &mlevels[len(mlevels)-1])
 		}
 
 		// Next are the file levels: L0 sub-levels followed by lower levels.

flushable.go

@@ -13,6 +13,7 @@ import (
 
 	"github.com/cockroachdb/errors"
 	"github.com/cockroachdb/pebble/internal/base"
+	"github.com/cockroachdb/pebble/internal/invalidating"
 	"github.com/cockroachdb/pebble/internal/invariants"
 	"github.com/cockroachdb/pebble/internal/keyspan"
 	"github.com/cockroachdb/pebble/internal/keyspan/keyspanimpl"
@@ -21,6 +22,9 @@ import (
 
 // flushable defines the interface for immutable memtables.
 type flushable interface {
+	// initMergingIterLevel initializes a mergingIterLevel for iteration over
+	// the flushable's point keys and range deletions.
+	initMergingIterLevel(ctx context.Context, o *IterOptions, mil *mergingIterLevel)
 	newIter(o *IterOptions) internalIterator
 	newFlushIter(o *IterOptions) internalIterator
 	newRangeDelIter(o *IterOptions) keyspan.FragmentIterator
@@ -209,6 +213,15 @@ func newIngestedFlushable(
 // TODO(sumeer): ingestedFlushable iters also need to plumb context for
 // tracing.
 
+func (s *ingestedFlushable) initMergingIterLevel(
+	ctx context.Context, o *IterOptions, mil *mergingIterLevel,
+) {
+	li := newLevelIter(ctx, *o, s.comparer, s.newIters, s.slice.Iter(), manifest.Level(0), internalIterOpts{})
+	li.initRangeDel(&mil.rangeDelIter)
+	mil.levelIter = li
+	mil.iter = invalidating.MaybeWrapIfInvariants(li)
+}
+
 // newIter is part of the flushable interface.
 func (s *ingestedFlushable) newIter(o *IterOptions) internalIterator {
 	var opts IterOptions
@@ -244,8 +257,6 @@ func (s *ingestedFlushable) constructRangeDelIter(
 }
 
 // newRangeDelIter is part of the flushable interface.
-// TODO(bananabrick): Using a level iter instead of a keyspan level iter to
-// surface range deletes is more efficient.
 //
 // TODO(sumeer): *IterOptions are being ignored, so the index block load for
 // the point iterator in constructRangeDeIter is not tracked.

mem_table.go

@@ -6,6 +6,7 @@ package pebble
 
 import (
 	"bytes"
+	"context"
 	"fmt"
 	"os"
 	"sync"
@@ -252,6 +253,15 @@ func (m *memTable) apply(batch *Batch, seqNum uint64) error {
 	return nil
 }
 
+func (m *memTable) initMergingIterLevel(
+	ctx context.Context, o *IterOptions, mil *mergingIterLevel,
+) {
+	*mil = mergingIterLevel{
+		iter:         m.newIter(o),
+		rangeDelIter: m.newRangeDelIter(o),
+	}
+}
+
 // newIter is part of the flushable interface. It returns an iterator that is
 // unpositioned (Iterator.Valid() will return false). The iterator can be
 // positioned via a call to SeekGE, SeekLT, First or Last.