> [!NOTE] Merge Append > <table> > <tr> > <td width="25%"><img src="assets/ex_mergeappend.png"></td> > <td>Merges multiple pre-sorted result sets while preserving the sort order.</td> > </tr> > </table> > > ```sql > -- Merging two sorted index scans with UNION ALL + ORDER BY > EXPLAIN (ANALYZE, COSTS, BUFFERS, VERBOSE) > SELECT * FROM (SELECT id FROM animals WHERE id < 100 ORDER BY id) s1 > UNION ALL > SELECT * FROM (SELECT id FROM animals WHERE id >= 100 AND id < 200 ORDER BY id) s2 > ORDER BY id; > ``` > > ```text > Merge Append (cost=1.58..14.31 rows=199 width=4) (actual time=1.008..1.018 rows=199 loops=1) > Sort Key: animals.id > Buffers: shared hit=6 > -> Index Only Scan using animals_pkey on public.animals (...) > -> Index Only Scan using animals_pkey on public.animals animals_1 (...) > ``` > > <table> > <tr> > <td rowspan="5" width="25%"><img src="assets/ex_merge_append.svg"></td> > <td><b>Performance</b></td><td>High performance as it uses a tournament tree or heap to merge pre-sorted inputs.</td> > </tr> > <tr><td><b>Factors</b></td><td>Number and size of input datasets, and the cost of the child sort/index scan nodes.</td></tr> > <tr><td><b>Cost</b></td><td>Sum of sorted subquery costs.</td></tr> > <tr><td><b>Operates on</b></td><td><a href="Structures/Result Set">Result Set</a></td></tr> > <tr><td><b>Workloads</b></td><td><a href="Workloads/LWLock/Buffers/BufferContent">LWLock: BufferContent</a>, <a href="Workloads/IPC/Parallel/AppendReady">IPC: AppendReady</a></td></tr> > <tr><td colspan="3"><b>Description</b>: Merges sorted results from multiple subqueries.</td></tr> > </table>