fix common-phrase optimization field scope

[sanikolaev]

Related issue #4128

Changes in this PR prevent boolean optimizations from collapsing field-scoped phrases into an all-fields phrase. The fix keeps optimizations within the same field/zone spec and preserves the spec when new phrase nodes are created.

Changes by area

1) Add field/zone spec hashing helper

File: src/sphinxquery/transform_commonkeywords.cpp

What changed

• Added a small helper HashSpec(const XQLimitSpec_t & tSpec) that hashes:
  • m_dFieldMask
  • m_iFieldMaxPos
  • m_bZoneSpan
  • m_dZones (if any)

Why

• The common keyword/phrase transforms group and compare nodes using hashes of phrase content. Without including the field/zone spec, phrases scoped to different fields could be treated as equivalent, which led to cross-field collapsing.

Effect

• Phrases with different field/zone constraints no longer collide in hash-based grouping.

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2) Seed phrase and subphrase hashes with the spec

File: src/sphinxquery/transform_commonkeywords.cpp

What changed

• sphHashPhrase() now starts its hash from HashSpec(pNode->m_dSpec) rather than 0.
• sphHashSubphrases() now seeds each subphrase hash with HashSpec(pNode->m_dSpec).

Why

• Ensures phrase equality checks are spec-aware, preventing different field scopes from being optimized together.

Effect

• Boolean optimization for common keywords only applies when the field/zone spec matches.

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3) Include spec in common-phrase head/tail bigram hashes

File: src/sphinxquery/transform_commonkeywords.cpp

What changed

• The head/tail bigram hashes for common-phrase detection are now seeded with HashSpec(...) before hashing words.

Why

• Common-phrase detection uses these bigram hashes to find shared heads/tails across phrases. Without spec in the hash, phrases from different fields could be grouped together.

Effect

• Common-phrase factoring only happens among phrases with the same field/zone constraints.

---

4) Preserve field/zone spec when building new phrase nodes

File: src/sphinxquery/transform_commonkeywords.cpp

What changed

• In MakeTransformCommonPhrase(), new nodes now use the original node’s XQLimitSpec_t instead of XQLimitSpec_t():
  • pCommonPhrase
  • pNewOr
  • pNewPhrase

Why

• The previous default constructor yields “all fields,” which erased the original field scope during optimization and widened matches.

Effect

• Optimized trees preserve the original field/zone scope, fixing the incorrect broadening.

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Behavior impact

• Queries like (@f "a b") | (@f2 "a b") will no longer match rows where the phrase appears only in unrelated fields.
• Optimizations remain intact when phrases share the same field/zone spec.

Files touched

• src/sphinxquery/transform_commonkeywords.cpp

[klirichek]

As a side note:
'explain query' by default (by design?) doesn't apply boolean optimization. So, results of 'explain' and 'select + show plan' are different. See output of test 397 (added cases).