> [!SubtransSLRU] > <table><tr><td rowspan="4" width="25%"><img src="assets/wl_lw_subtransslru.png"></td><td><b>Wait Category</b></td><td>LWLock</td></tr><tr><td><b>Mechanism</b></td><td><code>SubtransSLRULock</code></td></tr><tr><td><b>Operations</b></td><td><a href="Operations/Table/SeqScan">SeqScan</a>, <a href="Operations/Tuple/ModifyTable">ModifyTable</a></td></tr><tr><td><b>Resources</b></td><td><a href="Resources/RAM">Memory</a>, <a href="Resources/CPU">CPU</a></td></tr><tr><td colspan="3">Contention on the <b>Sub-transaction Mapping Layer</b>. Because Postgres uses sub-transactions for <code>SAVEPOINT</code>s and PL/pgSQL <code>EXCEPTION</code> blocks, the engine must be able to quickly determine if a given row version (created by a sub-transaction) belongs to a committed parent transaction.</td></tr></table>