而Contact不去重直接合并。 所以Contact当然比较快了。所以如果你不用去重的时候,尽量用Contact做合并吧。 在SQL里面:Union uses ‘UNION’ while Concat uses ‘UNION ALL’. Union removes duplicates.Concat does not. So, they produce different results if the sources either contain any items in common, or...
We can "unroll" the union definition to obtain three subtypes of the type packet, namely redirect, parameter and unreachable, which correspond, respectively to instances of packet where the union field is a ih gwaddr, ih pptr or ih pmtu. Thus, as shown in Figure 1(b), each of the ...
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