> [!NOTE] Index Scan > <table> > <tr> > <td width="25%"><img src="assets/ex_indexscan.png"></td> > <td>A targeted lookup that first traverses an index structure to find specific Tuple IDs (TIDs) matching the filter. It then performs random I/O to fetch those specific rows from the table heap. Efficient for high-selectivity queries where only a small percentage of the table is needed.</td> > </tr> > </table> > > ```sql > -- Point lookup by Primary Key > EXPLAIN (ANALYZE, COSTS, BUFFERS, VERBOSE) > SELECT * FROM animals WHERE id = 100; > ``` > > ```text > Index Scan using animals_pkey on public.animals (cost=1.29..9.30 rows=1 width=27) (actual time=1.028..1.029 rows=1 loops=1) > Output: id, name, species_id, created_at > Index Cond: (animals.id = 100) > Buffers: shared hit=1 read=2 > Planning Time: 2.519 ms > Execution Time: 1.062 ms > ``` > > <table> > <tr> > <td rowspan="5" width="25%"><img src="assets/ex_index_scan.svg"></td> > <td><b>Performance</b></td><td>High performance for low-selectivity queries (returning few rows).</td> > </tr> > <tr><td><b>Factors</b></td><td>Index height, disk I/O for random heap lookups.</td></tr> > <tr><td><b>Cost</b></td><td><code>index cost + cpu_index_tuple_cost * index entries scanned</code></td></tr> > <tr><td><b>Operates on</b></td><td><a href="Structures/Index">Index</a></td></tr> > <tr><td><b>Workloads</b></td><td><a href="Workloads/IO/DataFile/DataFileRead">IO: DataFileRead</a>, <a href="Workloads/IO/DataFile/DataFilePrefetch">IO: DataFilePrefetch</a>, <a href="Workloads/LWLock/Buffers/BufferContent">LWLock: BufferContent</a>, <a href="Workloads/LWLock/Buffers/BufferMapping">LWLock: BufferMapping</a>, <a href="Workloads/Lock/relation">Lock: relation</a>, <a href="Workloads/Lock/page">Lock: page</a></td></tr> > <tr><td colspan="3"><b>Description</b>: Scans a table using an index.</td></tr> > </table>