release-20.2: backfill: clear bound account after index KV has been written

pkg/sql/backfill/backfill.go

@@ -518,6 +518,12 @@ func (ib *IndexBackfiller) Close(ctx context.Context) {
 	}
 }
 
+// Clear releases the allocations on the IndexBackfiller's bound account,
+// prepping it for reuse.
+func (ib *IndexBackfiller) Clear(ctx context.Context) {
+	ib.boundAccount.Clear(ctx)
+}
+
 // initCols is a helper to populate column metadata of an IndexBackfiller. It
 // populates the cols and colIdxMap fields.
 func (ib *IndexBackfiller) initCols(desc *tabledesc.Immutable) {
@@ -628,6 +634,9 @@ func (ib *IndexBackfiller) init(
 
 // BuildIndexEntriesChunk reads a chunk of rows from a table using the span sp
 // provided, and builds all the added indexes.
+// The method accounts for the memory used by the index entries for this chunk
+// using the memory monitor associated with ib. It is the callers responsibility
+// to clear the associated bound account when appropriate.
 func (ib *IndexBackfiller) BuildIndexEntriesChunk(
 	ctx context.Context,
 	txn *kv.Txn,
@@ -789,5 +798,10 @@ func (ib *IndexBackfiller) RunIndexBackfillChunk(
 		return nil, ConvertBackfillError(ctx, tableDesc, batch)
 	}
 
+	// After the chunk entries have been written, we must clear the bound account
+	// tracking the memory usage for the chunk.
+	entries = nil
+	ib.Clear(ctx)
+
 	return key, nil
 }

pkg/sql/rowexec/indexbackfiller.go

@@ -186,6 +186,13 @@ func (ib *indexBackfiller) runChunk(
 			return nil, ib.wrapDupError(ctx, err)
 		}
 	}
+
+	// After the index KVs have been copied to the underlying BulkAdder, we can
+	// free the memory which was accounted when building the index entries of the
+	// current chunk.
+	entries = nil
+	ib.Clear(ctx)
+
 	if knobs.RunAfterBackfillChunk != nil {
 		if err := ib.adder.Flush(ctx); err != nil {
 			return nil, ib.wrapDupError(ctx, err)