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what's hsitoric MVCC Snapshot?

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Here are the comments in src/include/utils/snapshot.h.
        /*
* For normal MVCC snapshot this contains the all xact IDs that are in
* progress, unless the snapshot was taken during recovery in which case
* it's empty. For historic MVCC snapshots, the meaning is inverted, i.e.
* it contains *committed* transactions between xmin and xmax.
*
* note: all ids in xip[] satisfy xmin <= xip[i] < xmax
*/
TransactionId *xip;
I can't understand the historic MVCC snapshots? can you give me a scenario
to describe this?

jacktby@xxxxxxxxx

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