Postrges implementation of introspectIndexSchema is incorrect

The ordering of the index column names was added in #3031479: TaxonomyVocabularyHierarchyUpdateTest::testTaxonomyUpdateParents() is prone to breaking on PostgreSQL →

This comment → offers and explanation for why the there explains why the ORDER BY index_name was added.

Exactly! More specifically, the pgsql query to get the index column names does not return them in the order they were defined in the schema array, and if we add any kind of ordering to the query, it still won't match the initial schema order.

@quietone nice sleuthing. I still feel that we should try to get the columns in the order they are in the index. Sorting by column name is incorrect because the order in the index is significant.