Performance: LDBC query30 regressed in 0.18.x

[adsharma]

------------------------------------------------------------------------------------- benchmark: 30 tests -------------------------------------------------------------------------------------
Name (time in ms)               Min                 Max                Mean            StdDev              Median               IQR            Outliers       OPS            Rounds  Iterations
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
0.15.3.log:test_benchmark_query30     242.3995 (234.29)   268.5763 (124.34)   251.3555 (203.72)   7.5211 (37.04)    249.0095 (210.20)   10.5158 (116.06)        7;0    3.9784 (0.00)         20           1
0.16.0.log:test_benchmark_query30     251.2776 (239.34)   276.4616 (121.02)   261.6479 (204.16)   7.5762 (37.78)    259.6273 (208.88)   10.1605 (95.81)         8;0    3.8219 (0.00)         20           1
0.17.0.log:test_benchmark_query30     266.5449 (230.76)   298.2371 (110.57)   277.8978 (179.42)   7.8000 (29.11)    275.2066 (194.25)   11.1292 (60.58)         6;0    3.5984 (0.01)         20           1
0.18.0.log:test_benchmark_query30     714.0764 (611.99)   721.9423 (302.64)   717.2040 (505.94)   2.2148 (16.53)    717.0413 (515.95)   3.3097 (23.05)         7;0    1.3943 (0.00)         20           1
0.18.3.log:test_benchmark_query30     659.3717 (526.32)   668.8924 (221.43)   663.0676 (394.57)   2.5989 (7.45)     662.4803 (433.02)   3.5198 (15.66)         6;0    1.5081 (0.00)         20           1

[adsharma]

Profile on 0.18.0

lbug> profile         MATCH (c:Comment)-[:commentHasCreator]->(creator:Person),
                    (c)-[:replyOfPost]->(post:Post)-[:postHasCreator]->(creator)
              RETURN COUNT(DISTINCT c.ID) > 0 AS has_self_reply;
┌─────────────────────────────────────┐
│┌───────────────────────────────────┐│
││           Physical Plan           ││
│└───────────────────────────────────┘│
└─────────────────────────────────────┘
┌─────────────────────────────────────┐
│             PROFILE[23]             │
│   -------------------------------   │
│   -------------------------------   │
│         NumOutputTuples: 0          │
│   -------------------------------   │
│       ExecutionTime: 0.000000       │
└──────────────────┬──────────────────┘
┌──────────────────┴──────────────────┐
│        RESULT_COLLECTOR[22]         │
│   -------------------------------   │
│     Expressions: has_self_reply     │
│   -------------------------------   │
│         NumOutputTuples: 1          │
│   -------------------------------   │
│       ExecutionTime: 0.040000       │
└──────────────────┬──────────────────┘
┌──────────────────┴──────────────────┐
│           PROJECTION[21]            │
│   -------------------------------   │
│     Expressions: has_self_reply     │
│   -------------------------------   │
│         NumOutputTuples: 1          │
│   -------------------------------   │
│       ExecutionTime: 1.451000       │
└──────────────────┬──────────────────┘
┌──────────────────┴──────────────────┐
│         AGGREGATE_SCAN[20]          │
│   -------------------------------   │
│   Aggregate: COUNT(DISTINCT c.ID)   │
│   -------------------------------   │
│         NumOutputTuples: 1          │
│   -------------------------------   │
│       ExecutionTime: 0.006000       │
└──────────────────┬──────────────────┘
┌──────────────────┴──────────────────┐
│       AGGREGATE_FINALIZE[19]        │
│   -------------------------------   │
│   Aggregate: COUNT(DISTINCT c.ID)   │
│   -------------------------------   │
│         NumOutputTuples: 0          │
│   -------------------------------   │
│       ExecutionTime: 2.415000       │
└──────────────────┬──────────────────┘
┌──────────────────┴──────────────────┐
│            AGGREGATE[18]            │
│   -------------------------------   │
│   Aggregate: COUNT(DISTINCT c.ID)   │
│   -------------------------------   │
│         NumOutputTuples: 0          │
│   -------------------------------   │
│      ExecutionTime: 14.995000       │
└──────────────────┬──────────────────┘
┌──────────────────┴──────────────────┐
│           PROJECTION[17]            │
│   -------------------------------   │
│          Expressions: c.ID          │
│   -------------------------------   │
│        NumOutputTuples: 4752        │
│   -------------------------------   │
│      ExecutionTime: 17.495000       │
└──────────────────┬──────────────────┘
┌──────────────────┴──────────────────┐
│         HASH_JOIN_PROBE[16]         │
│   -------------------------------   │
│             Keys: c._ID             │
│   -------------------------------   │────────────────────┐
│        NumOutputTuples: 4752        │                    │
│   -------------------------------   │                    │
│     ExecutionTime: 5350.302000      │                    │
└──────────────────┬──────────────────┘                    │
┌──────────────────┴──────────────────┐ ┌──────────────────┴──────────────────┐
│             FLATTEN[1]              │ │         HASH_JOIN_BUILD[15]         │
│   -------------------------------   │ │   -------------------------------   │
│   -------------------------------   │ │             Keys: c._ID             │
│      NumOutputTuples: 2052169       │ │   -------------------------------   │
│   -------------------------------   │ │        NumOutputTuples: 4752        │
│     ExecutionTime: 2115.712000      │ │   -------------------------------   │
│                                     │ │       ExecutionTime: 7.083000       │
└──────────────────┬──────────────────┘ └──────────────────┬──────────────────┘
┌──────────────────┴──────────────────┐ ┌──────────────────┴──────────────────┐
│         SCAN_NODE_TABLE[0]          │ │           PROJECTION[14]            │
│   -------------------------------   │ │   -------------------------------   │
│           Tables: Comment           │ │         Expressions: c._ID          │
│              Alias: c               │ │   -------------------------------   │
│          Properties: c.ID           │ │        NumOutputTuples: 4752        │
│   -------------------------------   │ │   -------------------------------   │
│      NumOutputTuples: 2052169       │ │       ExecutionTime: 9.567000       │
│   -------------------------------   │ │                                     │
│      ExecutionTime: 86.086000       │ │                                     │
└─────────────────────────────────────┘ └──────────────────┬──────────────────┘
                                        ┌──────────────────┴──────────────────┐
                                        │            INTERSECT[13]            │
                                        │   -------------------------------   │
                                        │             Key: c._ID              │
                                        │   -------------------------------   │────────────────────┬───────────────────────────────────────┐
                                        │        NumOutputTuples: 4752        │                    │                                       │
                                        │   -------------------------------   │                    │                                       │
                                        │     ExecutionTime: 2481.289000      │                    │                                       │
                                        └──────────────────┬──────────────────┘                    │                                       │
                                        ┌──────────────────┴──────────────────┐ ┌──────────────────┴──────────────────┐ ┌──────────────────┴──────────────────┐
                                        │             FLATTEN[12]             │ │         INTERSECT_BUILD[5]          │ │         INTERSECT_BUILD[9]          │
                                        │   -------------------------------   │ │   -------------------------------   │ │   -------------------------------   │
                                        │   -------------------------------   │ │          Keys: creator._ID          │ │           Keys: post._ID            │
                                        │      NumOutputTuples: 1003605       │ │           Payloads: c._ID           │ │           Payloads: c._ID           │
                                        │   -------------------------------   │ │   -------------------------------   │ │   -------------------------------   │
                                        │      ExecutionTime: 895.500000      │ │       NumOutputTuples: 10337        │ │       NumOutputTuples: 196180       │
                                        │                                     │ │   -------------------------------   │ │   -------------------------------   │
                                        │                                     │ │      ExecutionTime: 122.732000      │ │      ExecutionTime: 445.077000      │
                                        └──────────────────┬──────────────────┘ └──────────────────┬──────────────────┘ └──────────────────┬──────────────────┘
                                        ┌──────────────────┴──────────────────┐ ┌──────────────────┴──────────────────┐ ┌──────────────────┴──────────────────┐
                                        │         SCAN_REL_TABLE[11]          │ │            PROJECTION[4]            │ │            PROJECTION[8]            │
                                        │   -------------------------------   │ │   -------------------------------   │ │   -------------------------------   │
                                        │       Tables: postHasCreator        │ │      Expressions: creator._ID       │ │        Expressions: post._ID        │
                                        │   Direction: (creator)<-[]-(post)   │ │                c._ID                │ │                c._ID                │
                                        │   -------------------------------   │ │   -------------------------------   │ │   -------------------------------   │
                                        │      NumOutputTuples: 1003605       │ │      NumOutputTuples: 2052169       │ │      NumOutputTuples: 1011420       │
                                        │   -------------------------------   │ │   -------------------------------   │ │   -------------------------------   │
                                        │      ExecutionTime: 18.730000       │ │      ExecutionTime: 29.751000       │ │      ExecutionTime: 675.922000      │
                                        └──────────────────┬──────────────────┘ └──────────────────┬──────────────────┘ └──────────────────┬──────────────────┘
                                        ┌──────────────────┴──────────────────┐ ┌──────────────────┴──────────────────┐ ┌──────────────────┴──────────────────┐
                                        │         SCAN_NODE_TABLE[10]         │ │          SCAN_REL_TABLE[3]          │ │          SCAN_REL_TABLE[7]          │
                                        │   -------------------------------   │ │   -------------------------------   │ │   -------------------------------   │
                                        │           Tables: Person            │ │      Tables: commentHasCreator      │ │         Tables: replyOfPost         │
                                        │           Alias: creator            │ │    Direction: (creator)<-[]-(c)     │ │      Direction: (post)<-[]-(c)      │
                                        │   -------------------------------   │ │   -------------------------------   │ │   -------------------------------   │
                                        │        NumOutputTuples: 9892        │ │      NumOutputTuples: 2052169       │ │      NumOutputTuples: 1011420       │
                                        │   -------------------------------   │ │   -------------------------------   │ │   -------------------------------   │
                                        │       ExecutionTime: 0.115000       │ │      ExecutionTime: 25.579000       │ │      ExecutionTime: 250.841000      │
                                        └─────────────────────────────────────┘ └──────────────────┬──────────────────┘ └──────────────────┬──────────────────┘
                                                                                ┌──────────────────┴──────────────────┐ ┌──────────────────┴──────────────────┐
                                                                                │         SCAN_NODE_TABLE[2]          │ │         SCAN_NODE_TABLE[6]          │
                                                                                │   -------------------------------   │ │   -------------------------------   │
                                                                                │           Tables: Person            │ │            Tables: Post             │
                                                                                │           Alias: creator            │ │             Alias: post             │
                                                                                │   -------------------------------   │ │   -------------------------------   │
                                                                                │        NumOutputTuples: 9892        │ │      NumOutputTuples: 1003605       │
                                                                                │   -------------------------------   │ │   -------------------------------   │
                                                                                │       ExecutionTime: 0.055000       │ │       ExecutionTime: 1.211000       │
                                                                                └─────────────────────────────────────┘ └─────────────────────────────────────┘

[adsharma]

Profile on 0.17.0

lbug> profile         MATCH (c:Comment)-[:commentHasCreator]->(creator:Person),
                    (c)-[:replyOfPost]->(post:Post)-[:postHasCreator]->(creator)
              RETURN COUNT(DISTINCT c.ID) > 0 AS has_self_reply;
┌─────────────────────────────────────┐
│┌───────────────────────────────────┐│
││           Physical Plan           ││
│└───────────────────────────────────┘│
└─────────────────────────────────────┘
┌─────────────────────────────────────┐
│             PROFILE[23]             │
│   -------------------------------   │
│   -------------------------------   │
│         NumOutputTuples: 0          │
│   -------------------------------   │
│       ExecutionTime: 0.000000       │
└──────────────────┬──────────────────┘
┌──────────────────┴──────────────────┐
│        RESULT_COLLECTOR[22]         │
│   -------------------------------   │
│     Expressions: has_self_reply     │
│   -------------------------------   │
│         NumOutputTuples: 1          │
│   -------------------------------   │
│       ExecutionTime: 0.079000       │
└──────────────────┬──────────────────┘
┌──────────────────┴──────────────────┐
│           PROJECTION[21]            │
│   -------------------------------   │
│     Expressions: has_self_reply     │
│   -------------------------------   │
│         NumOutputTuples: 1          │
│   -------------------------------   │
│       ExecutionTime: 0.205000       │
└──────────────────┬──────────────────┘
┌──────────────────┴──────────────────┐
│         AGGREGATE_SCAN[20]          │
│   -------------------------------   │
│   Aggregate: COUNT(DISTINCT c.ID)   │
│   -------------------------------   │
│         NumOutputTuples: 1          │
│   -------------------------------   │
│       ExecutionTime: 0.013000       │
└──────────────────┬──────────────────┘
┌──────────────────┴──────────────────┐
│       AGGREGATE_FINALIZE[19]        │
│   -------------------------------   │
│   Aggregate: COUNT(DISTINCT c.ID)   │
│   -------------------------------   │
│         NumOutputTuples: 0          │
│   -------------------------------   │
│       ExecutionTime: 7.589000       │
└──────────────────┬──────────────────┘
┌──────────────────┴──────────────────┐
│            AGGREGATE[18]            │
│   -------------------------------   │
│   Aggregate: COUNT(DISTINCT c.ID)   │
│   -------------------------------   │
│         NumOutputTuples: 0          │
│   -------------------------------   │
│      ExecutionTime: 14.260000       │
└──────────────────┬──────────────────┘
┌──────────────────┴──────────────────┐
│           PROJECTION[17]            │
│   -------------------------------   │
│          Expressions: c.ID          │
│   -------------------------------   │
│        NumOutputTuples: 4752        │
│   -------------------------------   │
│      ExecutionTime: 17.405000       │
└──────────────────┬──────────────────┘
┌──────────────────┴──────────────────┐
│         HASH_JOIN_PROBE[16]         │
│   -------------------------------   │
│             Keys: c._ID             │
│   -------------------------------   │────────────────────┐
│        NumOutputTuples: 4752        │                    │
│   -------------------------------   │                    │
│     ExecutionTime: 5345.349000      │                    │
└──────────────────┬──────────────────┘                    │
┌──────────────────┴──────────────────┐ ┌──────────────────┴──────────────────┐
│             FLATTEN[1]              │ │         HASH_JOIN_BUILD[15]         │
│   -------------------------------   │ │   -------------------------------   │
│   -------------------------------   │ │             Keys: c._ID             │
│      NumOutputTuples: 2052169       │ │   -------------------------------   │
│   -------------------------------   │ │        NumOutputTuples: 4752        │
│     ExecutionTime: 2117.649000      │ │   -------------------------------   │
│                                     │ │       ExecutionTime: 8.415000       │
└──────────────────┬──────────────────┘ └──────────────────┬──────────────────┘
┌──────────────────┴──────────────────┐ ┌──────────────────┴──────────────────┐
│         SCAN_NODE_TABLE[0]          │ │           PROJECTION[14]            │
│   -------------------------------   │ │   -------------------------------   │
│           Tables: Comment           │ │         Expressions: c._ID          │
│              Alias: c               │ │   -------------------------------   │
│          Properties: c.ID           │ │        NumOutputTuples: 4752        │
│   -------------------------------   │ │   -------------------------------   │
│      NumOutputTuples: 2052169       │ │      ExecutionTime: 11.062000       │
│   -------------------------------   │ │                                     │
│      ExecutionTime: 86.095000       │ │                                     │
└─────────────────────────────────────┘ └──────────────────┬──────────────────┘
                                        ┌──────────────────┴──────────────────┐
                                        │            INTERSECT[13]            │
                                        │   -------------------------------   │
                                        │             Key: c._ID              │
                                        │   -------------------------------   │────────────────────┬───────────────────────────────────────┐
                                        │        NumOutputTuples: 4752        │                    │                                       │
                                        │   -------------------------------   │                    │                                       │
                                        │     ExecutionTime: 2596.684000      │                    │                                       │
                                        └──────────────────┬──────────────────┘                    │                                       │
                                        ┌──────────────────┴──────────────────┐ ┌──────────────────┴──────────────────┐ ┌──────────────────┴──────────────────┐
                                        │             FLATTEN[12]             │ │         INTERSECT_BUILD[5]          │ │         INTERSECT_BUILD[9]          │
                                        │   -------------------------------   │ │   -------------------------------   │ │   -------------------------------   │
                                        │   -------------------------------   │ │          Keys: creator._ID          │ │           Keys: post._ID            │
                                        │      NumOutputTuples: 1003605       │ │           Payloads: c._ID           │ │           Payloads: c._ID           │
                                        │   -------------------------------   │ │   -------------------------------   │ │   -------------------------------   │
                                        │     ExecutionTime: 3464.879000      │ │       NumOutputTuples: 10337        │ │       NumOutputTuples: 196178       │
                                        │                                     │ │   -------------------------------   │ │   -------------------------------   │
                                        │                                     │ │      ExecutionTime: 120.427000      │ │      ExecutionTime: 445.535000      │
                                        └──────────────────┬──────────────────┘ └──────────────────┬──────────────────┘ └──────────────────┬──────────────────┘
                                        ┌──────────────────┴──────────────────┐ ┌──────────────────┴──────────────────┐ ┌──────────────────┴──────────────────┐
                                        │         SCAN_REL_TABLE[11]          │ │            PROJECTION[4]            │ │            PROJECTION[8]            │
                                        │   -------------------------------   │ │   -------------------------------   │ │   -------------------------------   │
                                        │       Tables: postHasCreator        │ │      Expressions: creator._ID       │ │        Expressions: post._ID        │
                                        │   Direction: (post)-[]->(creator)   │ │                c._ID                │ │                c._ID                │
                                        │   -------------------------------   │ │   -------------------------------   │ │   -------------------------------   │
                                        │      NumOutputTuples: 1003605       │ │      NumOutputTuples: 2052169       │ │      NumOutputTuples: 1011420       │
                                        │   -------------------------------   │ │   -------------------------------   │ │   -------------------------------   │
                                        │     ExecutionTime: 1027.198000      │ │      ExecutionTime: 29.920000       │ │      ExecutionTime: 673.293000      │
                                        └──────────────────┬──────────────────┘ └──────────────────┬──────────────────┘ └──────────────────┬──────────────────┘
                                        ┌──────────────────┴──────────────────┐ ┌──────────────────┴──────────────────┐ ┌──────────────────┴──────────────────┐
                                        │         SCAN_NODE_TABLE[10]         │ │          SCAN_REL_TABLE[3]          │ │          SCAN_REL_TABLE[7]          │
                                        │   -------------------------------   │ │   -------------------------------   │ │   -------------------------------   │
                                        │            Tables: Post             │ │      Tables: commentHasCreator      │ │         Tables: replyOfPost         │
                                        │             Alias: post             │ │    Direction: (creator)<-[]-(c)     │ │      Direction: (post)<-[]-(c)      │
                                        │   -------------------------------   │ │   -------------------------------   │ │   -------------------------------   │
                                        │      NumOutputTuples: 1003605       │ │      NumOutputTuples: 2052169       │ │      NumOutputTuples: 1011420       │
                                        │   -------------------------------   │ │   -------------------------------   │ │   -------------------------------   │
                                        │       ExecutionTime: 1.538000       │ │      ExecutionTime: 26.028000       │ │      ExecutionTime: 242.026000      │
                                        └─────────────────────────────────────┘ └──────────────────┬──────────────────┘ └──────────────────┬──────────────────┘
                                                                                ┌──────────────────┴──────────────────┐ ┌──────────────────┴──────────────────┐
                                                                                │         SCAN_NODE_TABLE[2]          │ │         SCAN_NODE_TABLE[6]          │
                                                                                │   -------------------------------   │ │   -------------------------------   │
                                                                                │           Tables: Person            │ │            Tables: Post             │
                                                                                │           Alias: creator            │ │             Alias: post             │
                                                                                │   -------------------------------   │ │   -------------------------------   │
                                                                                │        NumOutputTuples: 9892        │ │      NumOutputTuples: 1003605       │
                                                                                │   -------------------------------   │ │   -------------------------------   │
                                                                                │       ExecutionTime: 0.056000       │ │       ExecutionTime: 1.427000       │
                                                                                └─────────────────────────────────────┘ └─────────────────────────────────────┘

[adsharma]

What Got Reversed

The physical plan uses an INTERSECT to combine two paths to c._ID:

1. Left of INTERSECT — traverse postHasCreator to get c._ID:

     (creator:Person) ← postHasCreator — (post:Post) ← replyOfPost — (c:Comment)
   

   (producing c._ID) — this is the replyOfPost → postHasCreator leg

2. Right side of INTERSECT (two build inputs):

   • commentHasCreator gives (c:Comment) ← commentHasCreator — (creator:Person) → (c._ID)
   • replyOfPost gives (c:Comment) ← replyOfPost — (post:Post) → (c._ID)

   These are intersected with the left leg to ensure the creator from the first path matches.

Within that INTERSECT, the left leg (FLATTEN[12]) feeds from postHasCreator relationship scans. Here's what changed:

┌─────────┬──────────────────────────────────────────────────────────┬──────────────────┬──────────────────┐
│ Version │ postHasCreator traversal                                 │ Start node       │ Start node size  │
├─────────┼──────────────────────────────────────────────────────────┼──────────────────┼──────────────────┤
│ 0.17.0  │ (post)-[]->(creator) — outgoing, as written in the query │ Post             │ 1,003,605 tuples │
├─────────┼──────────────────────────────────────────────────────────┼──────────────────┼──────────────────┤
│ 0.18.0  │ (creator)<-[]-(post) — reversed, incoming                │ Person (creator) │ 9,892 tuples     │
└─────────┴──────────────────────────────────────────────────────────┴──────────────────┴──────────────────┘

Why the Reversal Is Helpful

The query pattern is (post:Post)-[:postHasCreator]->(creator:Person). The naive approach (0.17.0) follows it literally — start from 1 million Post rows and walk each one's outgoing postHasCreator edge.

The optimizer in 0.18.0 realized it can traverse backwards: start from Person (only 9,892 rows) and walk incoming postHasCreator edges. Both produce the same result (1,003,605 relationship tuples), but:

• SCAN_REL_TABLE[11] dropped from 1,027 → 19 (55× faster) — far fewer starting nodes to look up in the adjacency index
• FLATTEN[12] dropped from 3,465 → 896 (3.9× faster) — less data to flatten from the smaller start set

The total INTERSECT subtree (including its build children) went from ~6,062 → ~3,398 units.

So the reversal is a genuine optimization — the 0.18.x planner correctly chose to start from the smaller side. But paradoxically, the benchmark says the overall query got 2.6× slower (278ms → 717ms), meaning the slowdown lives in infrastructure/overhead outside what these per-operator profile timings capture.

[adsharma]

Operator	0.17.0	0.18.0	Speedup
SCAN_REL_TABLE[11] (postHasCreator)	1,027	19	55×
FLATTEN[12]	3,465	896	3.9×
INTERSECT[13] total*	~6,062	~3,398	1.8×

Critical Path Analysis

The dominant pipeline is: Comment Scan → Flatten → Hash Join Probe → Aggregate

0.17.0: max(2,118, 6,062) + 5,350 + 15 = ~11,426   (bottleneck: FLATTEN[12] at 3,465)
0.18.0: max(2,118, 3,398) + 5,350 + 15 = ~8,763    (bottleneck: BUILD[9] at 917)

The 0.18.0 critical path is ~23% shorter — the profiles predict 0.18.0 should be faster.

[adsharma]

Root cause: traversal reversal removed morsel parallelism

The 0.18 plan is better for single-thread CPU work, but worse for wall-clock time because it removes the parallelism available to the 0.17 plan.

The reversed postHasCreator traversal changes the source from ~1,003,605 Post rows to 9,892 Person rows. Node/relationship scans are parallelized by node-group morsels. The Post source supplies many morsels, while the Person source fits in essentially one morsel, so expanding its relationships and the downstream INTERSECT build become mostly serial.

I reproduced this using the supplied LDBC harness with Connection.set_max_threads_for_exec, measuring conn.execute() only (not Polars conversion):

Threads	0.17.0 engine time	0.18.0 engine time
1	~449 ms	~395 ms
2	~268 ms	~374 ms
4	~172 ms	~340 ms
8	~145 ms	~347 ms
16	~140 ms	~339 ms

Thus the reversed plan is genuinely faster with one thread, but at 8+ threads 0.18 is ~2.4x slower, consistent with the reported regression. This is not Python/Polars result conversion: the extra time is already inside conn.execute().

Why the PROFILE analysis did not show it

ExecutionTime in the physical profile is accumulated across cloned per-thread operator instances, not wall-clock elapsed time. It therefore cannot be composed with max(...) + ... into a critical path. The profile correctly shows less aggregate work in the reversed traversal, but does not expose the lost morsel-level concurrency.

Fix directions

1. Make traversal/join cost account for effective parallelism (available source morsels) rather than cardinality alone.
2. Split source-node scans for relationship expansion into finer morsels, allowing the 9,892 Person start nodes to be shared across workers.

The second option preserves the lower-work plan; the first can choose the old direction where a serial source would dominate.

[adsharma]

#730 is related.