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   3    The transaction IDs of currently executing transactions are shown in
   4    <a class="link" href="view-pg-locks.html" title="53.13. pg_locks"><code class="structname">pg_locks</code></a>
   5    in columns <code class="structfield">virtualxid</code> and
   6    <code class="structfield">transactionid</code>.  Read-only transactions
   7    will have <code class="structfield">virtualxid</code>s but NULL
   8    <code class="structfield">transactionid</code>s, while both columns will be
   9    set in read-write transactions.
  10   </p><p>
  11    Some lock types wait on <code class="structfield">virtualxid</code>,
  12    while other types wait on <code class="structfield">transactionid</code>.
  13    Row-level read and write locks are recorded directly in the locked
  14    rows and can be inspected using the <a class="xref" href="pgrowlocks.html" title="F.31. pgrowlocks — show a table's row locking information">pgrowlocks</a>
  15    extension.  Row-level read locks might also require the assignment
  16    of multixact IDs (<code class="literal">mxid</code>;  see <a class="xref" href="routine-vacuuming.html#VACUUM-FOR-MULTIXACT-WRAPAROUND" title="24.1.5.1. Multixacts and Wraparound">Section 24.1.5.1</a>).
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