Preserve UNIQUE NULLS NOT DISTINCT

[ivgiuliani]

pgschema dump was silently dropping UNIQUE table constraints with NULLS NOT DISTINCT (something added by PG15+) by just emitting a plain UNIQUE (...) without preserving the NULLS NOT DISTINCT modifier.

E.g. given the following input

CREATE TABLE t (a int, b int, CONSTRAINT t_uniq UNIQUE NULLS NOT DISTINCT (a, b));

It'd produce the following (incorrect) output:

CREATE TABLE IF NOT EXISTS t (a int, b int, CONSTRAINT t_uniq UNIQUE (a, b));

This PR adds support for NULLS NOT DISTINCT so that the above produces the expected correct output inclusive of NULLS NOT DISTINCT.

As this is only supported by >=PG15, GetConstraintsForSchema has to use the to_jsonb trick like for is_period to work around the fact that pg_index.indnullsnotdistinct doesn't exist on older versions of postgres, so the read returns NULL in that case and COALESCE collapses it to false.

Fixes #412

[greptile-apps]

Greptile Summary

This PR fixes a silent data-loss bug where pgschema dump and plan dropped the NULLS NOT DISTINCT modifier from UNIQUE constraints (PG15+), causing the tool to emit semantically incorrect DDL. The fix threads a new NullsNotDistinct field through the IR, all three SQL-generation sites in table.go, and constraintsEqual, using the same to_jsonb PG14-compatibility trick already in place for conperiod.

Confidence Score: 4/5

Safe to merge; fix is correct and complete across all code paths, with only a missing dump-level regression test.

All changed paths are consistent and the PG14 compatibility approach (to_jsonb trick) is proven in this codebase. Only P2 finding: no testdata/dump test covering the original bug report scenario.

No files require special attention; the only gap is a missing dump test.

Important Files Changed

Filename	Overview
ir/queries/queries.sql	Adds LEFT JOIN to pg_index and uses the to_jsonb trick to read indnullsnotdistinct safely on PG14; pattern is consistent with the existing is_period column.
ir/queries/queries.sql.go	Generated file kept in sync with queries.sql; adds NullsNotDistinct sql.NullBool field and scan target correctly.
ir/inspector.go	Correctly reads NullsNotDistinct.Bool from the first row of each grouped constraint; consistent with IsTemporal handling.
ir/ir.go	Adds NullsNotDistinct bool field with correct JSON tag (omitempty) to the Constraint struct.
internal/diff/constraint.go	generateConstraintSQL now emits NULLS NOT DISTINCT modifier; constraintsEqual updated to detect modifier changes and trigger re-creation.
internal/diff/table.go	All three UNIQUE SQL generation paths (inline ADD COLUMN constraint, ADD CONSTRAINT for new table, ADD CONSTRAINT for existing table) updated consistently.
testutil/skip_list.go	New test correctly placed in skipListPG14 (NULLS NOT DISTINCT is PG15+), not in skipListPG14_17.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[pg_constraint c] -->|LEFT JOIN on conindid| B[pg_index i]
    B -->|to_jsonb trick| C{indnullsnotdistinct exists?}
    C -->|PG15+ yes| D[true/false]
    C -->|PG14 no / non-index constraint| E[COALESCE → false]
    D --> F[NullsNotDistinct sql.NullBool]
    E --> F
    F --> G[inspector.go Constraint.NullsNotDistinct]
    G --> H{SQL generation path}
    H -->|CREATE TABLE inline| I[generateConstraintSQL CONSTRAINT x UNIQUE NULLS NOT DISTINCT]
    H -->|ALTER TABLE ADD COLUMN inline| J[table.go L777 CONSTRAINT x UNIQUE NULLS NOT DISTINCT]
    H -->|ALTER TABLE ADD CONSTRAINT| K[table.go L887 and L1019 ADD CONSTRAINT x UNIQUE NULLS NOT DISTINCT]
    G --> L[constraintsEqual detects modifier change → re-create]

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[greptile-apps]

Missing dump test for NULLS NOT DISTINCT

The PR description says pgschema dump was the primary regression surface, yet no dump test (testdata/dump/) is added to verify that the dump command now emits NULLS NOT DISTINCT correctly when inspecting a live database. The diff test covers the planning path, but a dump test (raw.sql + pgschema.sql pair) would directly confirm the inspector → formatter round-trip and prevent regressions on the original reported bug.

[Copilot]

Pull request overview

This PR fixes pgschema dump and migration diffs so they correctly preserve PostgreSQL 15+ UNIQUE ... NULLS NOT DISTINCT table constraints instead of silently emitting a plain UNIQUE (...) and changing semantics.

Changes:

• Extend the constraints inspector query to read pg_index.indnullsnotdistinct in a PG14-safe way (to_jsonb + COALESCE).
• Add nulls_not_distinct to the IR constraint model and propagate it through the inspector.
• Update SQL generation and diff equality so UNIQUE NULLS NOT DISTINCT is emitted and detected as a meaningful change; add regression fixtures + integration test.

Reviewed changes

Copilot reviewed 18 out of 18 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
ir/queries/queries.sql	Adds nulls_not_distinct extraction via pg_index (PG14-safe) to the constraints query.
ir/queries/queries.sql.go	Regenerates query bindings and row struct to include nulls_not_distinct.
ir/ir.go	Adds NullsNotDistinct to the IR Constraint model.
ir/inspector.go	Populates Constraint.NullsNotDistinct from the query result.
internal/diff/constraint.go	Emits UNIQUE NULLS NOT DISTINCT for inline constraints and compares the flag in constraintsEqual.
internal/diff/table.go	Emits NULLS NOT DISTINCT in ALTER/inline constraint SQL generation paths.
cmd/dump/dump_integration_test.go	Adds integration coverage for issue #412.
testutil/skip_list.go	Skips the new PG15+ testcases on PG14.
testdata/dump/issue_412_unique_nulls_not_distinct/*	Adds dump regression test fixtures for issue #412.
testdata/diff/create_table/add_unique_constraint_nulls_not_distinct/*	Adds diff regression fixtures ensuring migrations preserve the modifier.

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[tianzhou]

LGTM.

Thanks for the contribution