# Prefetch
> Prefetch dramatically accelerates queries that involve I/O operations. It works by proactively reading buffers into operating system page cache or PostgreSQL's
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LLMS index: [llms.txt](/llms.txt)
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**since v0\.4\.0**
Prefetch dramatically accelerates queries that involve I/O operations. It works by proactively reading buffers into operating system page cache or PostgreSQL's buffer pool based on the actual needs of the index.
Prefetch helps overlap I/O with computation, as well as I/O with other I/O operations, significantly reducing the time spent waiting for buffers during query execution, thereby improving overall query performance.
> [!TIP]
> This feature is enabled by default. Any query can benefit from prefetching with VectorChord `0.4.0` and later versions.
Prior to version `0.4.0`, VectorChord used PostgreSQL's synchronous I/O interface. Since then, VectorChord utilizes asynchronous I/O interface on newer versions of PostgreSQL. The I/O interface used by VectorChord can be controlled via GUC parameters.
In the GUC parameters, the synchronous I/O interface is referred to as `read_buffer`, the asynchronous I/O interface as `read_stream`, and VectorChord's simulated asynchronous interface as `prefetch_buffer`.
For indexes much smaller than the memory, there is almost no difference among the three prefetch modes. `read_stream` accelerates all disk-based indexes; however, it is only available on PostgreSQL 17 and 18. To address this, VectorChord provides a simulated asynchronous I/O interface called `prefetch_buffer`, which is available on all PostgreSQL versions as a fallback for older versions. `prefetch_buffer` is less effective than `read_stream`.
| Prefetch Mode | Description | Supported PostgreSQL Versions |
| ----------------- | ------------------------------------------------------------------------------------------------ | --------------------------------- |
| `read_stream` | Asynchronous I/O interface in PostgreSQL | PostgreSQL 17, 18 |
| `prefetch_buffer` | VectorChord's simulated asynchronous I/O interface used as fallback on older PostgreSQL versions | PostgreSQL 13, 14, 15, 16, 17, 18 |
| `read_buffer` | Synchronous I/O interface in PostgreSQL | PostgreSQL 13, 14, 15, 16, 17, 18 |
To fully leverage this feature with `read_stream`, you need to enable asynchronous I/O on PostgreSQL 18. See also [PostgreSQL Tuning](/docs/vchord/usage/postgresql-tuning/#asynchronous-i-o).
## Reference
### Search Parameters (`vchordrq`)
**vchordrq**
#### `vchordrq.io_search`
**since v0\.4\.0**
- Description: This GUC parameter controls the I/O prefetching strategy for reading bit vectors in vector search, which can impact search performance on disk-based vectors.
- Type: string
- Domain: Depends on PostgreSQL version
- PostgreSQL 13, 14, 15, 16: `{"read_buffer", "prefetch_buffer"}`
- PostgreSQL 17, 18: `{"read_buffer", "prefetch_buffer", "read_stream"}`
- Default: Depends on PostgreSQL version
- PostgreSQL 13, 14, 15, 16: `prefetch_buffer`
- PostgreSQL 17, 18: `read_stream`
- Note:
- `read_buffer` indicates a preference for `ReadBuffer`.
- `prefetch_buffer` indicates a preference for both `PrefetchBuffer` and `ReadBuffer`.
- `read_stream` indicates a preference for `read_stream`.
#### `vchordrq.io_rerank`
**since v0\.4\.0**
- Description: This GUC parameter controls the I/O prefetching strategy for reading full precision vectors in vector search, which can significantly impact search performance on disk-based vectors.
- Type: string
- Domain: Depends on PostgreSQL version
- PostgreSQL 13, 14, 15, 16: `{"read_buffer", "prefetch_buffer"}`
- PostgreSQL 17, 18: `{"read_buffer", "prefetch_buffer", "read_stream"}`
- Default: Depends on PostgreSQL version
- PostgreSQL 13, 14, 15, 16: `prefetch_buffer`
- PostgreSQL 17, 18: `read_stream`
- Note:
- `read_buffer` indicates a preference for `ReadBuffer`.
- `prefetch_buffer` indicates a preference for both `PrefetchBuffer` and `ReadBuffer`.
- `read_stream` indicates a preference for `read_stream`.
### Search Parameters (`vchordg`)
**vchordg**
#### `vchordg.io_search`
**since v0\.5\.0**
- Description: This GUC parameter controls the I/O prefetching strategy for reading bit vectors in vector search, which can impact search performance on disk-based vectors.
- Type: string
- Domain: Depends on PostgreSQL version
- PostgreSQL 13, 14, 15, 16: `{"read_buffer", "prefetch_buffer"}`
- PostgreSQL 17, 18: `{"read_buffer", "prefetch_buffer", "read_stream"}`
- Default: Depends on PostgreSQL version
- PostgreSQL 13, 14, 15, 16: `prefetch_buffer`
- PostgreSQL 17, 18: `read_stream`
- Note:
- `read_buffer` indicates a preference for `ReadBuffer`.
- `prefetch_buffer` indicates a preference for both `PrefetchBuffer` and `ReadBuffer`.
- `read_stream` indicates a preference for `read_stream`.
#### `vchordg.io_rerank`
**since v0\.5\.0**
- Description: This GUC parameter controls the I/O prefetching strategy for reading full precision vectors in vector search, which can significantly impact search performance on disk-based vectors.
- Type: string
- Domain: Depends on PostgreSQL version
- PostgreSQL 13, 14, 15, 16: `{"read_buffer", "prefetch_buffer"}`
- PostgreSQL 17, 18: `{"read_buffer", "prefetch_buffer", "read_stream"}`
- Default: Depends on PostgreSQL version
- PostgreSQL 13, 14, 15, 16: `prefetch_buffer`
- PostgreSQL 17, 18: `read_stream`
- Note:
- `read_buffer` indicates a preference for `ReadBuffer`.
- `prefetch_buffer` indicates a preference for both `PrefetchBuffer` and `ReadBuffer`.
- `read_stream` indicates a preference for `read_stream`.
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Backlinks:
- [Overview](/docs/vchord/getting-started/overview/)
- [Rerank in Table](/docs/vchord/usage/rerank-in-table/)