snapshot question

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От qihua wu
Тема snapshot question
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Msg-id CAPoYto+ouSQoXmLEaXh0VdVV8Y1w54FDcsR4CcE-vBX8Zbgc2A@mail.gmail.com
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Ответы Re: snapshot question  (Bruce Momjian <bruce@momjian.us>)
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To get a snapshot we need to know xmin, xmax and xip. my question is why do we need them? Since every tuple in a page has t_xmin and t_xmax based on the page layout(https://www.postgresql.org/docs/current/storage-page-layout.html), if we has a snapshot at xip 12345, couldn't we just check every tuple whose t_xmin is lower than 12345 and already committed?

Thanks,
Qihua

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