RFC: Setting search defaults as an index option

Use cases

There are cases where a user wants to set a default search value (e.g. ivfflat.probes, hnsw.ef_search) value on a per-index basis, for example:

1. Data is partitioned across multiple tables (integer-based partition key, not related to vector) with indexes on each table. This avoids setting the value on a per-query basis
2. User has a single index, but doesn't want to set the search values on a per-query basis

Proposal

The proposal is to add new index options on the index AMs:

1. ivfflat: default_probes (default: NULL)
2. hnsw: default_ef_search (default: NULL)

PostgreSQL tracks when a GUC is changed. For this case, we can use the PGC_S_SESSION indicator to determine that GUC is changed in the database sessions and therefore we do not need to consult the index option.

The proposal would then use this heuristic:

if default_probes/default_ef_search is NULL
    use value from `ivfflat.probes`/ `hnsw.ef_search`
else
  if `ivfflat.probes`/`hnsw.ef_search` source is PGC_S_SESSION, use value of `ivfflat.probes`/`hnsw.ef_search`
  else use value from `default_probes`/`default_ef_search`

Alternatives

With additional work, we could set "smarter" automatic defaults for ivfflat.probes/hnsw.ef_search, but folks may still want to set their own index-specific defaults based on the above use cases.

Seeking feedback on this proposal.