- Spec.md: Feature specification for per-index default search parameters
- SpecResearch.md: Research on GUC source tracking and reloptions
- CodeResearch.md: Code analysis of existing pgvector patterns
- ImplementationPlan.md: 5-phase implementation plan

Implements RFC from pgvector#235

…-option_plan

[Search Defaults Index Option] Planning: Implementation plan for per-index search defaults

Phase 1 of implementing per-index default search parameters (Issue pgvector#235).

This commit extends the index options to support:
- default_probes for IVFFlat indexes
- default_ef_search for HNSW indexes

Changes:
- Added defaultProbes field to IvfflatOptions struct
- Added defaultEfSearch field to HnswOptions struct
- Registered new reloptions in IvfflatInit() and HnswInit()
- Added parsing entries in ivfflatoptions() and hnswoptions()
- Implemented getter functions IvfflatGetDefaultProbes() and HnswGetDefaultEfSearch()

The new options use 0 as a sentinel value meaning "unset", which allows
the existing GUC defaults to take precedence when no index default is specified.

This phase establishes the foundation; subsequent phases will add:
- GUC source detection for precedence resolution
- Scan integration to use effective values
- Cost estimation updates
- Test coverage

…-option_phase1

[Search Defaults Index Option] Phase 1: Index Option Infrastructure

Add IvfflatGetEffectiveProbes() and HnswGetEffectiveEfSearch() functions
that implement the precedence rules for resolving search parameters:
1. Explicit SET command takes precedence (source == PGC_S_SESSION)
2. Index default value if set (> 0)
3. GUC default value

These functions use PostgreSQL's find_option() API to detect whether a
GUC was explicitly set in the current session, enabling the per-index
default to take effect when users haven't explicitly overridden it.

Phase 2 of Search Defaults Index Option implementation.

…-option_phase2

[Search Defaults Index Option] Phase 2: GUC Resolution Logic

Update scan functions to use the new effective value resolution functions
instead of directly reading GUC variables:

- ivfflatbeginscan(): Use IvfflatGetEffectiveProbes(index) instead of
  ivfflat_probes for determining the number of probes to use

- GetScanItems(): Use HnswGetEffectiveEfSearch(index) instead of
  hnsw_ef_search for the HNSW search layer ef parameter

- ResumeScanItems(): Use HnswGetEffectiveEfSearch(index) instead of
  hnsw_ef_search for the batch size in iterative scans

This implements the precedence rules where:
1. Explicit SET command takes precedence
2. Index default value (if set via default_probes/default_ef_search option)
3. GUC default value (ivfflat.probes=1 or hnsw.ef_search=40)

Phase 3 of 235-search-defaults-index-option implementation.

…-option_phase3

[Search Defaults Index Option] Phase 3: Scan Integration

- ivfflatcostestimate(): Use IvfflatGetEffectiveProbes(index) instead of
  directly reading ivfflat_probes GUC, enabling index-specific default
  probes to influence cost estimates

- hnswcostestimate(): Use HnswGetEffectiveEfSearch(index) instead of
  directly reading hnsw_ef_search GUC, enabling index-specific default
  ef_search to influence cost estimates

This ensures the query planner uses the same effective search parameter
values that will be used at scan time, improving plan quality when
indexes have per-index search defaults configured.

Implementation notes:
- Moved index_close() after the effective value function calls to ensure
  the index relation remains valid when accessing rd_options
- All 14 existing regression tests pass

…-option_phase4

[Search Defaults Index Option] Implementation Phase 4: Cost Estimation

Phase 5: Comprehensive test coverage for index search parameter defaults

IVFFlat tests (test/sql/ivfflat_vector.sql):
- Test CREATE INDEX with default_probes option
- Test query using index default when GUC not explicitly SET
- Test explicit SET ivfflat.probes overrides index default
- Test RESET ivfflat.probes returns to using index default
- Test ALTER INDEX changes default_probes value
- Test ALTER INDEX RESET removes default_probes
- Test default_probes = 0 acts as unset (uses GUC default)
- Test invalid values rejected (default_probes = -1)

HNSW tests (test/sql/hnsw_vector.sql):
- Test CREATE INDEX with default_ef_search option
- Test query using index default when GUC not explicitly SET
- Test explicit SET hnsw.ef_search overrides index default
- Test RESET hnsw.ef_search returns to using index default
- Test ALTER INDEX changes default_ef_search value
- Test ALTER INDEX RESET removes default_ef_search
- Test default_ef_search = 0 acts as unset (uses GUC default)
- Test invalid values rejected (default_ef_search = -1)

All 14 regression tests pass.

…-option_phase5

[Search Defaults Index Option] Phase 5: Tests

- Create comprehensive Docs.md for the Search Defaults Index Option feature
- Update README.md with documentation for new index options
- Add CHANGELOG entry for the new feature

…-option_docs

[Search Defaults Index Option] Documentation