在SELECT子句中使用非索引字段时，为什么性能会降低？

Consider these three queries:

考虑这三个查询:

select sampleno from sample
    where markupdate > '1/1/2010'

select sampleno, markupdate from sample
    where markupdate > '1/1/2010'

select sampleno, markuptime from sample
    where markupdate > '1/1/2010'
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