BE-515: HashQL: Remove OFFSET 0 materialization fence from generated query

[indietyp]

🌟 What is the purpose of this PR?

HashQL: Remove OFFSET 0 materialization fence from generated SQL

Problem

Each PostgreSQL island is wrapped in a CROSS JOIN LATERAL subquery. The subquery previously included OFFSET 0 as a materialization fence to prevent PostgreSQL's planner from inlining the CASE tree expression and duplicating it across each composite field access in the SELECT list.

The planner's cost estimate favors the fenced plan (lower estimated cost), but empirical benchmarks show the opposite for queries with conditional CASE trees:

Query	Change (fenced → unfenced)
all_entities	−11% (improved)
filter_by_uuid	+10% (regressed)
diamond_cfg	−5% (improved)
sequential_filters	−72% (improved)
stress_test	−77% (improved)

For multi-body queries with conditional branches, the materialization overhead exceeds the duplication cost by 3–4×. The fence helps modestly for simple single-expression queries without branching (filter_by_uuid), but is catastrophic for the common case.

🚢 Has this modified a publishable library?

This PR:

• does not modify any publishable blocks or libraries, or modifications do not need publishing

📜 Does this require a change to the docs?

The changes in this PR:

• are internal and do not require a docs change

🕸️ Does this require a change to the Turbo Graph?

The changes in this PR:

• do not affect the execution graph

🛡 What tests cover this?

• Existing test coverage for the HashQL eval Postgres module.

❓ How to test this?

1. Checkout the branch.
2. Run the existing test suite.
3. Confirm all tests pass and query behavior is unchanged.

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[indietyp]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• BE-515: HashQL: Remove OFFSET 0 materialization fence from generated query #8650 👈 (View in Graphite)
• BE-514: HashQL: implement iterative adjustment passes in placement solver #8635
• BE-500, BE-501: HashQL: Unify mixed parameter resolution in data dependency analysis #8607
• BE-494: HashQL: Loop-breaker selection for recursive inlining #8600
• BE-482: HashQL: Remove logical not from MIR and fix postgres boolean lowering #8595
• BE-474: HashQL: Take into account terminator target eligibility in execution backend placement #8587
• BE-455: HashQL: Introduce evaluation orchestrator #8586
• BE-306: HashQL: PostgreSQL translation #8526
• BE-457: HashQL: MIR execution pipeline extensions for postgres compilation #8525
• BE-456: HashQL: Entity type model enrichment and graph store query extensions #8524
• BE-429: HashQL: Add island dependency graph with data requirement resolution #8501
• BE-428: HashQL: Simplify traversal tracking to path recording #8498
• BE-434: HashQL: Refactor entity storage path resolution with centralized schema mapping #8496
• BE-427: Add hashql-macros crate with procedural macros for ID types and symbols #8495
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.63%. Comparing base (dadf2c4) to head (9db238d).

Additional details and impacted files

@@                                            Coverage Diff                                            @@
##           bm/be-514-hashql-solver-iterate-forwardbackward-passes-to-convergence    #8650      +/-   ##
=========================================================================================================
+ Coverage                                                                  56.76%   63.63%   +6.86%     
=========================================================================================================
  Files                                                                        610     1163     +553     
  Lines                                                                      46028   129424   +83396     
  Branches                                                                    3077     4982    +1905     
=========================================================================================================
+ Hits                                                                       26130    82360   +56230     
- Misses                                                                     19644    46198   +26554     
- Partials                                                                     254      866     +612     

Flag	Coverage Δ
apps.hash-ai-worker-ts	1.40% <ø> (ø)
apps.hash-api	0.00% <ø> (ø)
blockprotocol.type-system	40.84% <ø> (?)
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rust.error-stack	90.87% <ø> (?)
rust.harpc-codec	84.70% <ø> (?)
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rust.harpc-tower	67.03% <ø> (?)
rust.harpc-types	0.00% <ø> (?)
rust.hash-codec	72.76% <ø> (?)
rust.hash-graph-api	2.52% <ø> (ø)
rust.hash-graph-authorization	62.34% <ø> (?)
rust.hash-graph-postgres-store	26.74% <ø> (?)
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rust.hash-graph-temporal-versioning	47.95% <ø> (?)
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rust.hash-graph-validation	83.45% <ø> (?)
rust.hashql-ast	87.23% <ø> (?)
rust.hashql-compiletest	28.31% <ø> (ø)
rust.hashql-core	82.25% <ø> (?)
rust.hashql-diagnostics	72.53% <ø> (?)
rust.hashql-hir	89.06% <ø> (?)
rust.hashql-mir	91.72% <ø> (ø)
rust.hashql-syntax-jexpr	94.05% <ø> (?)

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[codspeed-hq]

Merging this PR will not alter performance

✅ 80 untouched benchmarks

---

_{Comparing bm/be-515-hashql-remove-offset-0-materialization-fence-from-generated (9db238d) with bm/be-514-hashql-solver-iterate-forwardbackward-passes-to-convergence (dadf2c4)}