Feature Request: Support Column Comments in PostgresIntrospector

[MasaGon]

Feature Request: Support Column Comments in PostgresIntrospector

Summary

The PostgresIntrospector in kysely-data-api does not retrieve column comments, while the official Kysely PostgresIntrospector (v0.27+) does support this feature using col_description().

Current Behavior

The getTables() method in kysely-data-api/src/postgres-introspector.ts does not include column comments in the returned ColumnMetadata.

Current implementation:

.select([
  "a.attname as column",
  "a.attnotnull as not_null",
  "a.atthasdef as has_default",
  "c.relname as table",
  // ... other fields
])

Expected Behavior

Column comments should be retrieved, matching the behavior of Kysely's official PostgresIntrospector.

Kysely's implementation (v0.27+):

.select([
  // ... other fields
  sql<string | null>`col_description(a.attrelid, a.attnum)`.as('column_description'),
])

Then in #parseTableMetadata:

comment: it.column_description || undefined,

Use Case

This is essential for code generation tools (like kysely-codegen) that generate TypeScript interfaces with JSDoc comments from database schema comments.

Example workflow:

1. Add comment in migration: COMMENT ON COLUMN users.email IS 'User email address'
2. Run introspection
3. Generate types with comments for better IDE autocomplete

Proposed Solution

Add col_description() to the SELECT clause in postgres-introspector.ts:

.select([
  "a.attname as column",
  "a.attnotnull as not_null",
  "a.atthasdef as has_default",
  "c.relname as table",
  sql<string>`case when c.relkind = 'v' then true else false end`.as("is_view"),
  "ns.nspname as schema",
  "typ.typname as type",
  "dtns.nspname as type_schema",
  sql<string | null>`col_description(a.attrelid, a.attnum)`.as('column_description'),  // ← Add this
  
  // auto incrementing detection
  this.#db
    .selectFrom("pg_class")
    .select(sql`true`.as("auto_incrementing"))
    .whereRef("relnamespace", "=", "c.relnamespace")
    .where("relkind", "=", "S")
    .where("relname", "=", sql`c.relname || '_' || a.attname || '_seq'`)
    .as("auto_incrementing"),
])

And update #parseTableMetadata:

table.columns.push(
  Object.freeze({
    name: it.column,
    dataType: it.type,
    dataTypeSchema: it.type_schema,
    isNullable: !it.not_null,
    isAutoIncrementing: !!it.auto_incrementing,
    hasDefaultValue: it.has_default,
    comment: it.column_description || undefined,  // ← Add this
  })
);

References

• Kysely's PostgresIntrospector implementation: https://github.com/kysely-org/kysely/blob/master/src/dialect/postgres/postgres-introspector.ts
• Related Kysely issue: (New Feature) Introspect Table Comments kysely-org/kysely#1368 (Table comments feature request)
• PostgreSQL documentation: https://www.postgresql.org/docs/current/functions-info.html#FUNCTIONS-INFO-COMMENT-TABLE

Additional Context

I'm currently working around this limitation by manually querying pg_description, but native support would be cleaner and more consistent with Kysely's official dialects.

Would you accept a PR for this feature? I'm happy to contribute if this aligns with the project's direction.