Prefetch 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. 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
prefetch_buffer	VectorChord's simulated asynchronous I/O interface used as fallback on older PostgreSQL versions	PostgreSQL 13, 14, 15, 16, 17
read_buffer	Synchronous I/O interface in PostgreSQL	PostgreSQL 13, 14, 15, 16, 17

Furthermore, the Asynchronous I/O that will be introduced in PostgreSQL 18 will make read_stream more advantageous.

Reference

Search Parameters 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: {"read_buffer", "prefetch_buffer", "read_stream"}
• Default: Depends on PostgreSQL version
  • PostgreSQL 13, 14, 15, 16: prefetch_buffer
  • PostgreSQL 17: 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: {"read_buffer", "prefetch_buffer", "read_stream"}
• Default: Depends on PostgreSQL version
  • PostgreSQL 13, 14, 15, 16: prefetch_buffer
  • PostgreSQL 17: 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.