I remember going through this. Other backends will use pg_subtrans to know what transactions are in progress. They have to do the standard lookups to find the status of the parent transaction. The backend-local list of xids is needed so the commit can clean up those subtransaction xids so that later transactions don't have to use pg_subtrans.
Is there some solution whereby the common case (99.999% of transactions won't be subtransactoins) is fast, and the uncommon case of being in a subtransaction is slower?
Chris
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