The transaction IDs of currently executing transactions are shown in pg_locks in columns virtualxid and transactionid. Read-only transactions will have virtualxids but NULL transactionids, while both columns will be set in read-write transactions.
Some lock types wait on virtualxid, while other types wait on transactionid. Row-level read and write locks are recorded directly in the locked rows and can be inspected using the pgrowlocks extension. Row-level read locks might also require the assignment of multixact IDs (mxid; see Section 24.1.5.1).
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