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Oracle Hash Group By

原创 Linux操作系统 作者:chncaesar 时间:2013-11-08 23:01:09 0 删除 编辑

Starting with 10g, Oracle introduced “Hash Group By” as the default grouping by mechanism.

Oracle employs a hash algorithm to calculate the hash value for each row based on the GROUP BY columns. This allows rows with identical column values (and hence identical hash values) to be "brought together" without performing a full sort. And then the aggregation is performed upon each group. The hash group by does not guarantee the order of the output, one should use “order by” clause to ensure the order.


select time_id, sum(amount_sold) from sales group by time_id;



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