sql: presplit temp hash sharded index

fixes #76686
we presplit hash sharded index before backflling it. but with the new
mvcc index backfiller, we also create a temp index to take care of write
traffic when the origin index is being backfilled. We need to presplit
the temp index as well.

Release note: None
Release justification: this needed by the mvcc index backfiller

pkg/ccl/logictestccl/testdata/logic_test/partitioning_hash_sharded_index

@@ -281,4 +281,4 @@ t_presplit  116       t_presplit_idx_member_id  /Table/116/2/"seattle"/3   /Tabl
 t_presplit  116       t_presplit_idx_member_id  /Table/116/2/"seattle"/4   /Table/116/2/"seattle"/5
 t_presplit  116       t_presplit_idx_member_id  /Table/116/2/"seattle"/5   /Table/116/2/"seattle"/6
 t_presplit  116       t_presplit_idx_member_id  /Table/116/2/"seattle"/6   /Table/116/2/"seattle"/7
-t_presplit  116       t_presplit_idx_member_id  /Table/116/2/"seattle"/7   /Max
+t_presplit  116       t_presplit_idx_member_id  /Table/116/2/"seattle"/7   /Table/116/3/"new york"/0

pkg/sql/logictest/testdata/logic_test/hash_sharded_index

@@ -840,7 +840,7 @@ t_hash_pre_split  139       t_hash_pre_split_idx_b  /Table/139/2/3  /Table/139/2
 t_hash_pre_split  139       t_hash_pre_split_idx_b  /Table/139/2/4  /Table/139/2/5
 t_hash_pre_split  139       t_hash_pre_split_idx_b  /Table/139/2/5  /Table/139/2/6
 t_hash_pre_split  139       t_hash_pre_split_idx_b  /Table/139/2/6  /Table/139/2/7
-t_hash_pre_split  139       t_hash_pre_split_idx_b  /Table/139/2/7  /Max
+t_hash_pre_split  139       t_hash_pre_split_idx_b  /Table/139/2/7  /Table/139/3/0
 
 subtest test_default_bucket_count
 

pkg/sql/schema_changer.go

@@ -2985,9 +2985,9 @@ func (sc *SchemaChanger) shouldSplitAndScatter(
 		return false
 	}
 
-	if m.Adding() && idx.IsSharded() && !idx.IsTemporaryIndexForBackfill() {
+	if m.Adding() && idx.IsSharded() {
 		if sc.mvccCompliantAddIndex {
-			return m.Backfilling()
+			return m.Backfilling() || (idx.IsTemporaryIndexForBackfill() && m.DeleteOnly())
 		}
 		return m.DeleteOnly()
 	}

pkg/sql/schema_changer_test.go

@@ -7484,8 +7484,8 @@ func TestHashShardedIndexRangePreSplit(t *testing.T) {
 	defer log.Scope(t).Close(t)
 	ctx := context.Background()
 
-	getShardedIndexRanges := func(tableDesc *tabledesc.Mutable, kvDB *kv.DB, codec keys.SQLCodec) ([]kv.KeyValue, error) {
-		indexSpan := tableDesc.IndexSpan(codec, descpb.IndexID(2))
+	getShardedIndexRanges := func(tableDesc *tabledesc.Mutable, kvDB *kv.DB, codec keys.SQLCodec, indexID descpb.IndexID) ([]kv.KeyValue, error) {
+		indexSpan := tableDesc.IndexSpan(codec, indexID)
 		ranges, err := kvDB.Scan(
 			ctx,
 			keys.RangeMetaKey(keys.MustAddr(indexSpan.Key)),
@@ -7524,23 +7524,29 @@ CREATE TABLE t.test_split(a INT PRIMARY KEY, b INT NOT NULL);
 	)
 
 	runBeforePreSplitting = func(tableDesc *tabledesc.Mutable, kvDB *kv.DB, codec keys.SQLCodec) error {
-		ranges, err := getShardedIndexRanges(tableDesc, kvDB, codec)
-		if err != nil {
-			return err
-		}
-		if len(ranges) != 0 {
-			return errors.Newf("expected 0 ranges but found %d", len(ranges))
+		// 2 is the id for the new index
+		// 3 is the id for temp index for backfilling
+		for id := range []int{2, 3} {
+			ranges, err := getShardedIndexRanges(tableDesc, kvDB, codec, descpb.IndexID(id))
+			if err != nil {
+				return err
+			}
+			if len(ranges) != 0 {
+				return errors.Newf("expected 0 ranges but found %d", len(ranges))
+			}
 		}
 		return nil
 	}
 
 	runAfterPreSplitting = func(tableDesc *tabledesc.Mutable, kvDB *kv.DB, codec keys.SQLCodec) error {
-		ranges, err := getShardedIndexRanges(tableDesc, kvDB, codec)
-		if err != nil {
-			return err
-		}
-		if len(ranges) != 8 {
-			return errors.Newf("expected 8 ranges but found %d", len(ranges))
+		for _, id := range []int{2, 3} {
+			ranges, err := getShardedIndexRanges(tableDesc, kvDB, codec, descpb.IndexID(id))
+			if err != nil {
+				return err
+			}
+			if len(ranges) != 8 {
+				return errors.Newf("expected 8 ranges but found %d", len(ranges))
+			}
 		}
 		return nil
 	}