Shulou(Shulou.com)06/01 Report--

The application reports an error, the temporary table space is full, and the running data is interrupted.

Look at what the sql statement caused, and found that

Select count (1) * union***

After traversing two tens of millions of tables, accounting for about 5G of temporary table space, the cost of this statement has been reduced dozens of times after replacing union with union all.

So what's the difference between union and union all?

Union: joins two result sets, excluding duplicate rows, and sorts the default rules

Union All: joins two result sets, including duplicate rows, without sorting

Union consumes a lot more resources than union all because duplicate rows need to be removed and sorted.

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