To satisfy the rule that writes to a table must happen at a timestamp greater than the table's creation timestamp, ddl operations must happen in a separate applyOps before non-ddl operations. As the only ddl operations allowed in a multi-document transaction are creating columns and creating indexes, we can move all ddl operations to before all non-ddl operations rather than having to interleave them. The relative order of ddl operations must be preserved, though (so that creating an index on a newly created collection works).