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This section explains the specific meaning of xmax in snapshot through source code.

I. Xmax

The previous section mentioned that PostgreSQL takes snapshots via the txid_current_snapshot() function, in the format xmin : xmax : xip_list, where xmax should be understood as txid + 1 of the last completed transaction (COMMITTED/ABORTED).

See PG source code below:

SnapshotGetSnapshotData(Snapshot snapshot){ /* xmax is always latestCompletedXid + 1 */ xmax = ShmemVariableCache->latestCompletedXid; Assert(TransactionIdIsNormal(xmax)); TransactionIdAdvance(xmax); /* initialize xmin calculation with xmax */ globalxmin = xmin = xmax; ... snapshot->xmax = xmax; ... return snapshot;}

xmax is always latestCompletedXid + 1, txid + 1 of the last completed transaction (COMMITTED/ABORTED)(ShmemVariableCache->latestCompletedXid + 1).

II. References

PostgreSQL Source Code

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