[SQL] Why must the exporting transaction remain active when importing a snapshot in PostgreSQL?

I'm working with PostgreSQL and using pg_export_snapshot() to create a snapshot in one transaction, which I then import into another transaction using SET TRANSACTION SNAPSHOT. I noticed that PostgreSQL requires the original (exporting) transaction to stay active while the snapshot is being used by the importing transaction.

Could someone explain why the exporting transaction must remain open for the snapshot to be valid in the importing transaction? Specifically, what is the technical reason behind this requirement?

Any insights into how PostgreSQL handles snapshot consistency in this context would be helpful. Thank you!

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