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由 Ashwin Agrawal 提交于
For vacuum, page pruning and freezing to perform its job correctly on QE's, it needs to know globally what's the lowest dxid till any transaction can see in full cluster. Hence QD must calculate and send that info to QE. For this purpose using logic similar to one for calculating globalxmin by local snapshot. TMGXACT for global transactions serves similar to PROC and hence its leveraged to provide us lowest gxid for its snapshot. Further using its array, shmGxactArray, can easily find the lowest across all global snapshots and pass down to QE via snapshot. Adding unit test for createDtxSnapshot along with the change.
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