Oceanbase currently implements a cost based query rewriting (or-expansion). In fact, many advanced rewriting rules in the database (such as complex view merge and window function rewrite) need to be rewritten based on cost.
Or-expansion rewrites a query into several subqueries composed of union. This rewrite may provide better optimization space for each sub query, but it can also lead to execution of multiple sub queries, so this rewriting needs to be judged at cost.
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