> **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;
> ```
>
> 
>
> <!-- literal-explain-plan
> Captured EXPLAIN provenance for the adjacent reader-facing visual plan.
> Canonical capture metadata lives in scratch/actual_operation_plans.json.
>
> Merge Append (cost=0.58..14.30 rows=199 width=4) (actual time=0.006..0.018 rows=199 loops=1)
> Sort Key: animals.id
> Buffers: shared hit=6
> -> Index Only Scan using animals_pkey on public.animals (cost=0.29..6.02 rows=99 width=4) (actual time=0.002..0.005 rows=99 loops=1)
> Output: animals.id
> Index Cond: (animals.id < 100)
> Heap Fetches: 0
> Buffers: shared hit=3
> -> Index Only Scan using animals_pkey on public.animals animals_1 (cost=0.29..6.29 rows=100 width=4) (actual time=0.003..0.006 rows=100 loops=1)
> Output: animals_1.id
> Index Cond: ((animals_1.id >= 100) AND (animals_1.id < 200))
> Heap Fetches: 0
> Buffers: shared hit=3
> Planning:
> Buffers: shared hit=83
> Planning Time: 0.255 ms
> Execution Time: 0.054 ms
> -->
>
>
> <!--
> Raw-capture provenance — separate run.
> SQL, setup, dataset, settings, and scope: artifacts/chapter4_capture_matrix.json.
> Target: PostgreSQL 18.x companion fixture. Cache state: uncontrolled.
> Boundary: pg_wait_tracer backend execution root; client states are included when the chart shows them.
> Fidelity: exact pg_wait_tracer export. Not the adjacent EXPLAIN run; compare state shape, not durations.
> -->
>
> 
>
> <table>
> <tr>
> <td rowspan="2" 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>Cost</b></td><td>Sum of sorted subquery costs.</td></tr>
> </table>