perf: hash-adjacency overhaul — agehash + flat-array VertexEdgeArray

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

Author: jrgemignani

Makefile

@@ -127,6 +127,7 @@ OBJS = src/backend/age.o \
        src/backend/utils/ag_func.o \
        src/backend/utils/graph_generation.o \
        src/backend/utils/cache/ag_cache.o \
+       src/backend/utils/cache/agehash.o \
        src/backend/utils/load/ag_load_labels.o \
        src/backend/utils/load/ag_load_edges.o \
        src/backend/utils/load/age_load.o \
@@ -153,6 +154,7 @@ REGRESS = scan \
           graphid \
           agtype \
           agtype_hash_cmp \
+          agehash \
           catalog \
           cypher \
           expr \

age--1.7.0--y.y.y.sql

@@ -459,3 +459,13 @@ BEGIN
     END LOOP;
 END;
 $$;
+
+-- Internal selftest for the agehash open-addressing hashtable. Returns "OK"
+-- on success or "FAIL: ..." with a diagnostic message. Intended for the
+-- agehash regression test only.
+CREATE FUNCTION ag_catalog._agehash_self_test()
+    RETURNS text
+    LANGUAGE c
+    VOLATILE
+    PARALLEL UNSAFE
+AS 'MODULE_PATHNAME';

regress/expected/agehash.out

@@ -0,0 +1,14 @@
+/*
+ * agehash internal selftest.
+ *
+ * Exercises the Robin Hood open-addressing hashtable used by AGE's hot-path
+ * caches at boundary sizes (1, 7, 8, 9, 63, 64, 65, 1024, ...) and across
+ * grow thresholds. A success returns the literal "OK"; any failure returns
+ * "FAIL: <reason>" describing the offending case.
+ */
+SELECT ag_catalog._agehash_self_test();
+ _agehash_self_test 
+--------------------
+ OK
+(1 row)
+

regress/sql/agehash.sql

@@ -0,0 +1,9 @@
+/*
+ * agehash internal selftest.
+ *
+ * Exercises the Robin Hood open-addressing hashtable used by AGE's hot-path
+ * caches at boundary sizes (1, 7, 8, 9, 63, 64, 65, 1024, ...) and across
+ * grow thresholds. A success returns the literal "OK"; any failure returns
+ * "FAIL: <reason>" describing the offending case.
+ */
+SELECT ag_catalog._agehash_self_test();

sql/age_main.sql

@@ -86,6 +86,16 @@ CREATE FUNCTION ag_catalog._label_id(graph_name name, label_name name)
 PARALLEL SAFE
 AS 'MODULE_PATHNAME';
 
+-- Internal selftest for the agehash open-addressing hashtable. Returns "OK"
+-- on success or "FAIL: ..." with a diagnostic message. Intended for the
+-- agehash regression test only.
+CREATE FUNCTION ag_catalog._agehash_self_test()
+    RETURNS text
+    LANGUAGE c
+    VOLATILE
+    PARALLEL UNSAFE
+AS 'MODULE_PATHNAME';
+
 --
 -- utility functions
 --