[Pg-kit]: Fix unique constraint introspection returning zero rows

[kevin-courbet]

Description

Fixes a critical bug in PostgreSQL unique constraint introspection that causes drizzle-kit push to falsely detect missing constraints even when they exist in the database.

Problem

The current introspection query at pgSerializer.ts:1230-1236 uses information_schema.constraint_column_usage which creates a Cartesian product when joined with the columns table:

JOIN information_schema.constraint_column_usage AS ccu USING (constraint_schema, constraint_name)
JOIN information_schema.columns AS c ON c.table_schema = tc.constraint_schema
  AND tc.table_name = c.table_name AND ccu.column_name = c.column_name

This returns zero rows for multi-column unique constraints, causing drizzle-kit push to prompt users to add constraints that already exist.

Solution

Replace constraint_column_usage with key_column_usage (the standard PostgreSQL information_schema table for PRIMARY KEY and UNIQUE constraints) and properly join on:

• constraint_name
• constraint_schema
• table_name

Added ORDER BY kcu.ordinal_position to preserve column order in multi-column constraints.

Testing

Before Fix

-- Returns 0 rows for multi-column unique constraint
SELECT c.column_name FROM information_schema.table_constraints tc
JOIN information_schema.constraint_column_usage AS ccu USING (constraint_schema, constraint_name)
JOIN information_schema.columns AS c ON c.table_schema = tc.constraint_schema
  AND tc.table_name = c.table_name AND ccu.column_name = c.column_name
WHERE tc.table_name = 'address' AND tc.constraint_type = 'UNIQUE';
-- Result: 0 rows

After Fix

-- Returns all columns in the constraint
SELECT kcu.column_name FROM information_schema.table_constraints tc
JOIN information_schema.key_column_usage kcu
  ON tc.constraint_name = kcu.constraint_name
  AND tc.constraint_schema = kcu.constraint_schema
  AND tc.table_name = kcu.table_name
WHERE tc.table_name = 'address' AND tc.constraint_type = 'UNIQUE'
ORDER BY kcu.ordinal_position;
-- Result: 5 rows (all columns in the unique constraint)

Impact

• ✅ Fixes constraint detection for ALL multi-column unique constraints
• ✅ Eliminates false "add constraint" prompts in drizzle-kit push
• ✅ No breaking changes - only fixes broken introspection
• ✅ Compatible with all PostgreSQL versions

Related Issues

Closes #4490
Closes #4531
Closes #4921

Checklist

• Bug fix in drizzle-kit introspection
• Tested with real PostgreSQL database
• No breaking changes
• Commit signed

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Disclaimer: This PR was created with assistance from AI tools for bug diagnosis and documentation.