perf: hash-adjacency overhaul

[jrgemignani]

Replaces the per-graph adjacency map with a Robin Hood open-addressing hashtable (agehash) and an embedded flat-array edge list, removing the hottest dynahash path on IC1 and shrinking the largest hashtable AGE keeps. Stages land as one commit:

S1 MurmurHash3 fmix64 for graphid hashtables (replaces tag_hash)
S2 Precompute graphid hash; share across paired DFS lookups
S3 Replace ListGraphId adjacency with embedded flat-array
VertexEdgeArray (single palloc, contiguous iteration)
S4 Batched MLP lookup pipeline in add_valid_vertex_edges
S5/C1 agehash library: INLINE Robin Hood hashtable with
_with_hash API, freeze, iter, and a regress-only selftest
S5/C2 Wire global graph edge_hashtable through agehash;
drop edge_id from edge_entry (key lives in slot header);
AGEHASH_MAX_LOAD=0.85; MemoryContextAllocHuge for SF10+

Performance (SF3 LDBC SNB, 5 runs/3 warmup, vs clean master baseline_v2):

IC1 8,625 → 7,117 ms −17.49 % (the headline; hashtable-bound)
IU1 40 → 35 ms −11.86 % (heaviest update; lookup-bound)
IC sum 198,958 → 197,367 ms −0.80 % (suite-level noise)
IS sum 1,009 → 1,028 ms +1.86 % (IS3 jitter; sub-ms)
IU sum 77 → 72 ms −6.64 %
IC2/3/4/5/6/7/8/9/10/11/12: parity (within ±3.3 %, mostly ±1.5 %)

The VLE-DFS-heavy queries (IC3/5/6/9/11) sit at parity: with hash_search_with_hash_value at ≤1 % inclusive on their baseline flames, no hashtable swap can recover meaningful wall-time on them.

Memory: removing edge_id from edge_entry saves ~416 MB on SF3 and ~1.4 GB on SF10 for the global graph's edge_hashtable. Slot capacity uses MemoryContextAllocHuge so SF10+ edge tables can be built.

Adds:
src/backend/utils/cache/agehash.c, src/include/utils/agehash.h
regress/sql/agehash.sql + expected/agehash.out (boundary selftest)
_agehash_self_test() in both fresh-install and upgrade SQL

Tested on PostgreSQL 18.3 (REL_18_STABLE): all 35 regression tests pass (installcheck), warning-free build.

Co-authored-by: Claude noreply@anthropic.com

modified: Makefile
modified: age--1.7.0--y.y.y.sql
new file: regress/expected/agehash.out
new file: regress/sql/agehash.sql
modified: sql/age_main.sql
modified: src/backend/utils/adt/age_global_graph.c
modified: src/backend/utils/adt/age_vle.c
new file: src/backend/utils/cache/agehash.c
modified: src/include/utils/age_global_graph.h
new file: src/include/utils/agehash.h

[Copilot]

Pull request overview

This PR introduces a new high-performance open-addressing hashtable (agehash) and rewires AGE’s global-graph edge lookup + VLE traversal hot paths to reduce dynahash overhead and improve cache locality.

Changes:

• Add agehash (INLINE Robin Hood hashtable) plus SQL/regress self-test coverage.
• Replace global graph edge_hashtable (dynahash) with edge_table (agehash) and remove edge_id from the edge payload (key lives in the slot header).
• Replace per-vertex linked-list adjacency with embedded flat arrays and batch VLE edge lookups using precomputed graphid_hash.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
src/include/utils/agehash.h	Defines the agehash public/internal API contract, slot layout, and helper macros.
src/backend/utils/cache/agehash.c	Implements INLINE Robin Hood hashtable + iterator + SQL-callable self-test.
src/include/utils/age_global_graph.h	Introduces VertexEdgeArray, adds get_edge_entry_with_hash, and exposes graphid_hash/graphid_keyeq.
src/backend/utils/adt/age_global_graph.c	Switches global edge storage to agehash, embeds vertex adjacency arrays, implements graphid_hash, and updates accessors/freeing.
src/backend/utils/adt/age_vle.c	Reuses precomputed graphid hashes across paired lookups and batches adjacency processing for better MLP.
sql/age_main.sql	Adds SQL declaration for _agehash_self_test() for fresh installs.
age--1.7.0--y.y.y.sql	Adds _agehash_self_test() to the extension upgrade script.
regress/sql/agehash.sql	Adds regression test invoking the agehash self-test.
regress/expected/agehash.out	Expected output for the new agehash regression test.
Makefile	Builds agehash.o and adds agehash to the regression test list.

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