sql: fix flake in schemachange.TestWorkload

As soon as cockroachdb#79580 merged, it tickled flakes. These flakes were caused by
operations to alter the database which would build new mutable tables from
immutable tables and thus overwrite existing entries.

The test is very flakey without this patch.

Release note: None

pkg/sql/database.go

@@ -108,19 +108,28 @@ func (p *planner) forEachMutableTableInDatabase(
 		return err
 	}
 
+	// TODO(ajwerner): Rewrite this to not use the internalLookupCtx.
 	lCtx := newInternalLookupCtx(all.OrderedDescriptors(), dbDesc)
+	var droppedRemoved []descpb.ID
 	for _, tbID := range lCtx.tbIDs {
 		desc := lCtx.tbDescs[tbID]
 		if desc.Dropped() {
 			continue
 		}
-		mutable := tabledesc.NewBuilder(desc.TableDesc()).BuildExistingMutableTable()
-		schemaName, found, err := lCtx.GetSchemaName(ctx, desc.GetParentSchemaID(), desc.GetParentID(), p.ExecCfg().Settings.Version)
+		droppedRemoved = append(droppedRemoved, tbID)
+	}
+	descs, err := p.Descriptors().GetMutableDescriptorsByID(ctx, p.Txn(), droppedRemoved...)
+	if err != nil {
+		return err
+	}
+	for _, d := range descs {
+		mutable := d.(*tabledesc.Mutable)
+		schemaName, found, err := lCtx.GetSchemaName(ctx, d.GetParentSchemaID(), d.GetParentID(), p.ExecCfg().Settings.Version)
 		if err != nil {
 			return err
 		}
 		if !found {
-			return errors.AssertionFailedf("schema id %d not found", desc.GetParentSchemaID())
+			return errors.AssertionFailedf("schema id %d not found", d.GetParentSchemaID())
 		}
 		if err := fn(ctx, schemaName, mutable); err != nil {
 			return err