Percolator was designed to provide cross-row, cross-table ACID-compliant transactions via " snapshot isolation" since many threads on many machines need to transform the repository.
Right, when we do cross products, we are doing an expansion with respect to the first row.
算外积时，依第一行展开。
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In a cross join, each row from one table is combined with each row from another table, resulting in what is called a cross product or a Cartesian product.